By most accounts 2010 was the year of Tea Party. Not only did Republicans take control of the House, increase their numbers in the Senate, and gain ground virtually across the board, but conservative challengers had a good batting average against establishment favorites in GOP primaries.

This time the environment is less certain. Nobody knows how Republicans will fare in the general election, and there are fewer high-profile primaries along the lines of Marco Rubio versus Charlie Crist or Rand Paul versus Trey Grayson. Endangered Republican incumbents, like Arlen Specter of Pennsylvania and Robert Bennett in Utah, are even rarer.

Yet the potential for Tea Party upsets still exists. While the lead in Ohio’s Republican presidential primary seesawed back and forth between Mitt Romney and Rick Santorum, Buckeye State voters prepared one such surprise. Three-term Congresswoman Jean Schmidt unexpectedly lost her bid for re-nomination in the March 6 primary. She was beaten by Brad Wenstrup, a physician and Iraq War veteran who ran hard to her right and had never held elected office before.

Wenstrup followed a familiar script. Though outspent by the incumbent, he enjoyed outside support from Tea Party groups, whose super PACs helped narrow the money gap. He criticized Schmidt’s vote to raise the debt ceiling and made hay of a photo of the congresswoman smooching President Barack Obama, an image as endearing to conservative voters as Jimmy Carter kissing Soviet leader Leonid Brezhnev. While Schmidt had never been personally popular—she sneaked into office in a 2005 special election and was usually reelected only by pluralities in one of Ohio’s most Republican congressional districts—her defeat served as a reminder that grassroots activists can still make waves in GOP primaries.

“Wenstrup’s Tea Party victory in Ohio was completely under the radar of the national media, and demonstrates that the dynamics of the 2010 election are still at work,” wrote conservative fundraising pioneer Richard Viguerie. He argued that even unsuccessful primary challenges have a positive impact by motivating voters, identifying new activists, and preparing candidates for future campaigns. The title of Viguerie’s missive? Paraphrasing Clinton campaign guru James Carville, “It’s the primaries, stupid.”

So far most of the candidates heeding Viguerie’s admonition are running for House seats, like Wenstrup. Rep. Roscoe Bartlett, an 85-year-old staunch conservative from Maryland, faces no fewer than seven primary challengers due to his uncharacteristic vote to increase the debt ceiling—like Schmidt, he backed House Speaker John Boehner’s deal to resolve an impasse over the debt limit last summer.

Rep. Brian Bilbray of California has attracted a primary challenge from a semiconductor executive who brands the incumbent a “Republican in name only.” The Club for Growth is backing Evan Feinberg, a former Senate aide, in his bid to unseat Rep. Tim Murphy, a five-term GOP incumbent who represents the Pittsburgh suburbs.

But in the last election, the real Tea Party action was in Senate primaries. With activists cool to the GOP presidential field, one might have expected a bigger conservative imprint on this year’s races. At the request of party leadership, Sen. Jim DeMint of South Carolina, an important conservative kingmaker on Capitol Hill, isn’t supporting challenges to incumbent Republicans this time around. Sen. Mike Lee of Utah, one of the big Tea Party success stories of the midterms, is, but he’s only a freshman. Republicans hope to take the Senate in November, and many of them accept the conventional wisdom that Mitch McConnell would already be majority leader if it weren’t for a few upstart Tea Party candidates in 2010.

This year there is also a smaller pool of moderate Republican senators to mount primary challenges against. Olympia Snowe in Maine was an obvious target, but she declined to run for reelection. No one has declared a candidacy against Scott Brown, despite widespread Tea Party grumbling about the junior senator from Massachusetts.

That leaves just two incumbent Republican senators vulnerable to Tea Party challenge: Richard Lugar of Indiana and Orrin Hatch of Utah. The fact that either man has to defend himself before the GOP electorate is remarkable in itself. In the past, Republican primary voters have tended only to boot incumbents when they are severely tainted by scandal or to the left of their party on a wide range of issues. Although the Club for Growth focuses on economic policy, many of the candidates it has defeated were also social moderates.

Historically, at the Senate level many incumbents unseated by conservative primary challengers—think Clifford Chase or Jacob Javits—were outright liberals. And even in those cases, conservatives were not guaranteed success. Specter eked out a majority against Pat Toomey in 2004, before Toomey chased him out of the party six years later. Lincoln Chafee, a Rhode Island Republican senator so liberal that his 2005 American Conservative Union rating was identical to Hillary Clinton’s, won his primary in 2006. Ohio’s George Voinvoich inoculated himself from conservative challenges by compiling a pro-life voting record on abortion.

Compared to Chafee or Specter, Richard Lugar’s list of transgressions is small. But they have been more than enough to lure Indiana State Treasurer Richard Mourdock into the race: Lugar’s 1990s votes for the assault-weapons ban and the Brady bill, support for earmarks, opposition to congressional term limits, advocacy of taxpayer-funded embryonic stem-cell research—a position held by 2008 GOP presidential nominee John McCain—and the senator’s steadfast advocacy of the New Strategic Arms Reduction Treat (New START).

Ronald Reagan’s campaign against Gerald Ford received a boost from their disagreement over the Panama Canal treaty, so there is precedent. But compared to Specter versus Toomey, this is not exactly a referendum on bold colors versus pale pastels. In today’s political climate, however, it could be more than enough.

Lugar voted for the Troubled Asset Relief Program (TARP), a $700 billion Wall Street bailout. He supported the automobile bailout, consisting of car manufacturers receiving assistance from TARP funds originally approved for banks and eventually including a federal share in the ownership of General Motors. Lugar has been conciliatory toward Obama, voting to confirm both his Supreme Court nominees and agreeing to co-chair his inaugural committee. Neither would have been especially controversial moves in the past, but the Republican mood against such bipartisan niceties has hardened.

The Indiana senator’s resolve in defying the new conservative conventions has also hardened. By contrast, Orrin Hatch responded to a possible conservative primary challenge by ingratiating himself to the base. Hatch voted against both of Obama’s Supreme Court nominees, after voting for both parties’ judicial picks throughout most of his career. He was one of just a handful of Republicans to support Rand Paul’s budget proposal, which would have cut $500 billion in spending in a single year. Hatch signed on to the Cut, Cap, and Balance plan to reduce federal spending and address the debt ceiling impasse.

Lugar did none of these things, and he has been critical of his conservative detractors. He blasted his party’s intransigence on New START.  “At the moment, the Republican caucus is tied up in a situation where people don’t want to make choices,” he complained. The now 80-year-old senator seems genuinely baffled by Tea Party opposition. But unlike Hatch, Lugar—who last won reelection with 87 percent of the vote—doesn’t face a state convention dominated by conservative activists. Indiana Republicans will hold a primary.

So while there is precedent for unseating a nominally conservative incumbent Republican, Mourdock is the first to try to do so in a primary setting. This makes the odds tougher, but Mourdock is no Christine O’Donnell. Two years ago, he won reelection with 62 percent of the vote, topping the state Republican ticket. He rattles off his accomplishments as treasurer, such as earning $1 billion in investment income for the state and annually returning about 10 percent of his budget, and stresses his good working relationship with Gov. Mitch Daniels. Mourdock wanted Daniels to run for president. After the governor declined, Mourdock told me that he was intrigued by Michele Bachmann. He is currently neutral in the presidential race.

The party establishment in Indiana beat back a Tea Party attempt to keep Dan Coats from returning to his old Senate seat two years ago. But then the conservative vote was diluted, with two major conservative candidates—Marlin Stutzman, who went on to win a House seat, and former Congressman John Hostettler—together taking nearly 52 percent of the vote while Coats won less than 40 percent. This time, disenchanted conservatives are united behind Mourdock.

Mourdock says that Lugar isn’t in touch with today’s conservative Hoosiers. This is the same complaint Robert Bennett heard before Utah Republicans ousted him from the Senate in 2010. “What we’re really seeing here is the erosion of the patience that American voters have historically had,” Bennett recently told the Associated Press. “The comment being made … is, ‘Well, you haven’t gotten it done for 36 years. What makes you think you can finally get it done in the next six years?’”

Ted Cruz, the leading Tea Party candidate in the Texas GOP primary to replace retiring Sen. Kay Bailey Hutchinson, puts it differently. “Every Republican now says he is a conservative. Many will even get good ratings from conservative groups,” Cruz says. “But will they provide leadership? If I go to Washington and just have a good voting record [without leading], I will consider myself a failure.”

Conservative activists are also more discriminating than in the past. A single vote, such as support for TARP, can now be a career-killer come the Republican primary. This trend has its limits, of course. George Allen is expected to win the GOP nomination for his old Senate seat in Virginia, despite a spirited primary challenge from Jamie Radtke, a Tea Party activist who has assailed Allen’s support for earmarks and Bush-era boondoggles like the Medicare prescription drug benefit and No Child Left Behind. (Conveniently, Allen was out of the Senate by the time TARP came up for a vote, so he can safely say he opposed it.)

Sen. Lisa Murkowski lost the Republican primary two years ago, but was reelected as a write-in candidate. Lugar and Hatch are the only Republican incumbents to draw serious primary challengers this year. Politico describes the Lugar-Mourdock race as the Tea Party’s “first real, and maybe only legitimate, chance to knock off a sitting senator this cycle.”

In many respects, Lugar isn’t a terribly sympathetic candidate. During this campaign, he was forced to admit that he stays in hotels when he returns to Indiana and effectively doesn’t live in the state anymore. His arms-control advocacy has earned him an undeserved reputation for foreign-policy realism, as have his occasional muted criticisms of the wars for which he reliably votes. But Lugar, perhaps the most respected GOP wise man on the Senate Foreign Relations Committee, backed the Iraq War when it counted and will no doubt be up for seeing a sequel in Iran.

Even some conservatives have been influenced by the conventional wisdom that the Tea Party cost the Republicans control of the Senate in the last election. But in truth, GOP establishment candidates in the key races last time all had serious flaws as well—that’s why they ended up blowing large leads in their primaries—and only O’Donnell indisputably lost a race that the establishment favorite would likely have won. Nevertheless, Republicans this year, even those who like the Tea Party, want desperately to win.

This desire has clearly played out in the Republican presidential race, where primary voters were initially drawn to the red meat being served up by several implausible candidates only to return to safer, more familiar choices. It is also a factor in why there are fewer races like Lugar’s in 2012. If Lugar manages to hold on, his ability to deliver a landslide in November will be the main reason.

W. James Antle III is the associate editor of The American Spectator.