Excellent post by Seth Masket this morning about Veepstakes:

[Mitt] Romney, like [Sen. John] McCain, has been concerned about retaining the support of conservative activists given his own (recent) past with moderation, and he wanted a running mate who would remove some of the doubts about his ideological purity. [Rep. Paul] Ryan has become the darling of fiscal conservatives and many in the Tea Party movement over the past few years, and tapping him sent a signal to those parts of the GOP that Romney takes their concerns very seriously and is willing to tie his fate to theirs. Romney may well have made this decision on his own, but partisan actors certainly played a very powerful role in constraining that decision.

That sounds just about right. Parties are doing two things to influence the decision, it seems to me:

1. Vetoing people who violate important party orthodoxy; or, to put it a slightly different way, who disagree with core party groups on issues they care deeply about. Matt Yglesias asks about pro-choice Nevada Governor Brian Sandoval, “Is it really plausible that party activists would have abandoned a Romney/Sandoval ticket?” I think the answer to that is: Oh, definitely, or at least the risk is sufficient to prevent Romney from doing that. At the very least, it’s almost certain that pro-life delegates and outside groups would stage significant protests at the convention, including very possibly contesting the vice-presidential nomination (although I'm not sure whether the rules would allow them to actually nominate someone, they could almost certainly force a protest vote about the rules).

2. Parties create the information context in which the nominee is making the choice. That’s especially true for Republicans. Indeed, while we’ll learn more about the pick in the future (and, no, don’t trust what you hear now, which may be leaked as part of an electioneering effort), it seems at least plausible that Romney and the people around him might not have been fully aware of some of the risks involved in choosing the inexperienced, relatively unknown, Ryan and the issue positions he represented, because within the conservative information feedback loop Ryan was a much different figure than he was to the rest of the nation. Now, it’s not clear at all that Ryan will be a flop, but if Team Romney didn’t fully appreciate how risky he was, that may be because keeping the TV tuned to Fox News makes it harder to see that.

It’s also important to keep in mind the difference between candidates and candidacies. We might think of candidates, the individual politicians, as self-interested self-promoters entirely dedicated to winning office. But in order to run, they need to build candidacies, and in the last couple of decades that’s meant surrounding themselves with dedicated partisans whose interest in politics is at least as much about the party as it is about the current candidate.

So to go back to what Masket says: Yes, Mitt Romney had some freedom to choose, but it was severely constrained and influenced by what the Republican Party wanted — just as Barack Obama’s choice in 2008 was constrained and influenced by what the Democratic Party wanted.