Rand Paul tells the Washington Post‘s Dave Weigel that Thursday’s Republican presidential debate will pit him against rivals who “want to blow up the world.” He has reason to use stark language. After weeks of negative press, single-digit poll numbers, and lackluster fundraising, Senator Paul needs a “Giuliani moment”—something that will do for his campaign what a showdown with “America’s mayor” did for his father’s effort after the first debate of 2007.

In fact, Rand Paul has the opportunity to do much more than his father ever could. But he’s missing it: Rand’s “Giuliani moment” is the Iran deal, and it calls for action, not words.

Rand’s support for the deal would transform the politics of the Republican race at a stroke. He would also risk losing rather than gaining support—when the deal was announced, 30 percent of Republicans supported it, and those votes could have been Rand’s. Polls since then have been mixed and most indicate Republicans oppose the diplomatic effort, even overwhelmingly so.

But that’s where the Giuliani example is relevant: no pollster or campaign professional would have told Ron Paul to stand up to Giuliani like that—on an issue, national security and terrorism, that Giuliani owned and where Republican voters overwhelmingly disagreed with the Texas congressman. But Ron Paul did it anyway, and in so doing he pulled off something political pros usually believe is impossible or irrelevant: he changed voters’ minds.

He didn’t change nearly enough to win a single primary, of course, either in 2008 or in 2012. But Rand Paul starts from a stronger position and higher profile than his father had before that debate. If Rand dared, instead of being yet another single-term senator vying for the nomination, he could overnight become the most important player in the GOP on the biggest foreign-policy issue of the day. He’d get invited to every talk show as the one Republican with the audacity to side with the president to make a deal for peace. He’d be denounced, too, by every neocon outlet. In other words, he’d get the full-spectrum attention that Donald Trump now commands, knocking him out of the headlines, if not off the top of the polls.

He’d also be a legislative leader, a man Democrats and Republicans alike would have to court ahead of the vote on Iran. The pressure would be extraordinary, but if he stood by his support for the deal, he would have a polarizing and rallying effect, bringing other Republicans around—however many could be brought around—and shattering the GOP pro-war consensus that the neoconservative media has worked so hard to create.

Rand would perhaps even be in a position to demand legislative concessions from the Democrats and Obama; leadership would also be leverage. That might not be enough to defund Planned Parenthood—but consider what the public would be presented with if Rand Paul clearly supported the president on issues like Iran and sentencing reform but clearly separated from Obama and the Democrats on abortion and taxes. He’d give all voters something to think about, cutting across the left-right divide that has only meant defeat for Republicans in the last two presidential elections.

Instead, the strategy Rand’s team have devised for him is much more cautious, and its dividend so far has been dwindling support. But it doesn’t matter if a candidate drops into the single digits in the pre-primary season, and even if Rand’s fundraising could be better—Bush, Cruz, and Rubio beat him easily last quarter—he’s still a top-tier candidate. His playbook is to win on bread-and-butter Republican issues, demonstrating his support for tax cuts by literally cutting through the tax code with a chainsaw, courting Christian conservatives by calling for an end to federal funds for Planned Parenthood, keeping his libertarian supporters on board by opposing the NSA’s domestic surveillance, and reaching out to several groups at once—including libertarians, Christians, and some liberals—with criminal-justice reform.

His approach to two thorny questions—immigration and foreign policy—has been in line with this bread-and-butter strategy. There’s a vocal and somewhat large bloc of voters who say they want to restrict immigration, and while they may not tend not to vote in such a way as to prove their commitment—Tom Tancredo would have been a force in 2008 if they did, and John McCain would not have been the GOP nominee—an appeal to restrict immigration certainly won’t lose Rand many primary votes. By contrast, explicit noninterventionist appeals won’t win many: there aren’t legions of foreign-policy voters to begin with, and what few there are in the Republican Party are mostly hawks.

The logic of this play-it-safe strategy is impeccable. But it’s a logic that works against Rand Paul: after all, if voters want a bread-and-butter Republican, they have better options. Ted Cruz is a better orator, Marco Rubio is more charismatic, Scott Walker has an executive record. Christian conservatives aren’t going to choose Rand Paul over spiritual kin like Ben Carson and Mike Huckabee just because Rand, like the rest of the field, is antiabortion. (For one thing, the religious right suspects that in his bones Rand Paul is just too libertarian to fight till he bleeds against same-sex marriage.) Paul’s foreign-policy maneuvering, meanwhile, has the curious effect of leaving him the candidate least liked by hawks but no longer much loved by doves. What his campaign team has devised is actually a winning strategy for Scott Walker—or even Jeb Bush.

The dilemma for Rand is that his core supporters are with him because they believe he really is different from the rest on foreign policy, but to reach beyond that core he has to downplay the difference. Rand can’t do well as just Mr. Small Government because more Republicans seem to want Walker for that role, and Cruz can also compete for it. Rand’s core supporters don’t have a reason to go to another candidate, but there aren’t enough of them to make him the nominee. And there are fewer of them the more he triangulates.

His most devoted supporters dismiss those who are abandoning ship as “purists.” But there’s a spectrum: at one end are those who won’t settle for less than another Ron Paul, which Rand was never going to be. At the other are those who will stick with Rand no matter where he stands—either out of personal loyalty or out of an undying hope that he doesn’t really mean it when he falls in with GOP orthodoxy. But most of his potential support lies between the extremes, and that middle ground is where he’s failing to make the sale.

Going into the race Rand Paul could seemingly count on two iron-clad advantages: he would have plenty of funding, whether from Silicon Valley or from an army of small donors like those who backed his father. And he would have a ready-made bloc of activists composed of the more pragmatic of his father’s supporters and additional battalions of grassroots conservatives brought into the liberty movement by the senator’s ecumenical outreach. Paul’s struggle to win support either from the Silicon Valley or from a volume of grassroots supporters comparable to his father’s indicates there’s something about his campaign that fails to persuade the very people who should be most easily persuaded.

Rand’s strategy, unlike his father’s, is all about winning. But what no campaign professional likes to admit is that not every client has a chance of winning. The way the party’s attitudes presently stand, Republicans are not ready for Rand Paul. He could try to change the party so it’s ready for someone like him in 2020 or 2024. But instead he’s changing himself to be like the party of today. And he’s losing.

There are no shortcuts when it comes to altering the nation’s political course. If you want a very different foreign policy from what Republicans (and indeed Democrats) are used to, you can’t sneak it in by winning a single presidential election—just as you can’t stop abortion or erase the tax code with one November victory. A great deal of persuasion is necessary before the elections will follow, and the relationship between elections and public persuasion has to be mutually reinforcing: persuade more, then win more, then use your higher profile to persuade still more and win still more. That’s how you build a movement. It’s the only way.

Rand’s father faced a party that was even less ready for anyone with his principles. But he shook up the GOP and changed the way voters thought about his issues, to the point that his son, without any elected experience, could beat a well-funded establishment candidate in a 2010 Senate primary. There’s no question that without Ron Paul’s 2008 campaign, there would have been no Senator Paul.

Now Rand is in danger of reversing the momentum not only of his own campaign but of the liberty movement whose leadership he inherited. Ron Paul’s efforts in 2008 and 2012 helped set the stage for liberty Republicans like Justin Amash and Thomas Massie, as well as Rand, to win in 2010 and 2012. No similarly libertarian Republican won in 2014 or has yet to appear in prospect for 2016. This isn’t Rand’s fault: the issues environment has been more difficult for liberty candidates these past two years. But a movement needs leadership most of all when it faces adversity—someone who will risk his neck to ensure there are more Thomas Massies and fewer Tom Cottons sent to Washington in the future.

This Giuliani moment is a test of Rand Paul’s courage. If he fights for realistic diplomatic initiatives like the Iran deal, he may yet lose the nomination, but he’ll make political success for those with his principles—including himself—more likely in the future. Conversely, it will prove to be a mistake as well as a disgrace if Rand Paul is running for president to be someone rather than to do something—all the more so if who he’s trying to be is not who he is but who the other Republicans are.