John Sides quotes a reporter’s query (via Bernstein):

It seems that plenty of Republicans are mentioned as potential candidates in 4 years: Christie, Daniels, Rubio, Scott Walker, Jeb Bush… even Nikki Haley and Rand Paul. It seems far fewer Democrats are on the bench… there’s always Hillary, and some talk about Martin O’Malley and Andrew Cuomo, but I don’t hear too many more.

The reporter is correct that many more Republicans are mentioned as potential candidates, but that doesn’t mean that the presidential candidate “bench” is that much deeper in reality. The mentioning usually begins among movement conservative pundits and activists, and this is then picked up by reporters and pundits from outside the movement. The latter point to conservative “enthusiasm” for this or that politician, and then begin to pay more attention to the next “rising star” in the GOP, and that attention in turn causes more movement conservatives to promote these politicians as party leaders.

Sometimes the movement conservative promotion of these politicians is an expression of real interest in a new political talent, and sometimes it is an expression of dissatisfaction with the current party leadership or existing field of candidates, and sometimes it’s a bit of both. Unhappiness with Romney over the last year and a half led to the over-promotion of many young Republican politicians, almost all of whom probably would not have been mentioned so soon but for the fact that they provided a useful contrast with Romney. Jeb Bush was both the most experienced and one of the oldest politicians talked about as a possible presidential contender, and he was also the least likely to run of any of them.

The fact that so many first-term governors and senators’ names were being bandied about as possible presidential and vice presidential candidates was not proof of the depth of the Republican bench, but rather of its lack of depth. The point isn’t that these governors and senators won’t make plausible candidates someday in the future, but that movement conservatives treat them as plausible candidates almost as soon as they take office. Democrats don’t seem to do nearly as much of this, which creates the perception that they have very little new political talent. Of course, the losses in 2010 did to some would-be Democratic leaders what 2006 and 2008 did to the Republicans. For example, had Sestak won in Pennsylvania, as he might have done in a less pro-Republican election, he might be considered a possible presidential candidate for 2016 or 2020.

While movement conservative activists are often eager to identify and promote politicians that they like, their counterparts on the left don’t seem to find as many candidates that genuinely excite them. The Democrats may have a reasonably full “bench,” but the people on that “bench” are not promoted as much by activists on their side, and as a result they aren’t treated to a lot of flattering media coverage about their possible future ambitions. Elizabeth Warren is a favorite of activists, and many people outside of Massachusetts know much more about her than they do about other Democratic candidates running for election this year, which makes her unusual among new Democratic politicians.

Many of the people being mentioned for a future presidential run are the same people that some movement conservatives were touting as alternatives to the most recent candidate field as early as the start of 2011. Over-promoting new talent too quickly is a recurring problem for Republicans. That isn’t a mistake unique to them, but they seem to do it more often in recent years, in part because the Bush years decimated the ranks of their would-be future leaders. So they tend to fall back on the people elected in 2009 and 2010, because for the most part they don’t have many others available that aren’t closely associated with the Bush years. The promotion of Ryan to the national ticket is an odd exception to this, since he was closely associated with Bush-era policies and managed to come through as a leading member of the House GOP without being perceived as part of the problem.

Something else that creates the impression of a disparity between the two “benches” is that movement conservative activists and pundits tend to promote their preferred candidates regardless of their national viability or qualifications for higher office. It is easier to be included on the Republican “bench” because very few are ever kicked off the “bench” for lack of qualifications. Paul Ryan is a better example of this habit. Before early 2010, very few people outside Wisconsin and Washington had ever heard of Ryan, but over the next two years he was promoted as someone far more important than a leading member of the House GOP. Despite his lack of qualifications for the Presidency, Ryan boosters in conservative media promoted him as an possible presidential candidate, and then when that didn’t happen they kept pushing the idea of Ryan as a VP candidate. Of course, none of that guaranteed that Ryan would end up on the presidential ticket, but the effect of two years’ worth of building Ryan up made the idea of electing a young seven-term Congressman as President seem not only plausible but eminently desirable. There is simply nothing like this happening in the other party.