Greg Sargent has an excellent post today in which he calls out Mitt Romney for being the “just trust me” candidate. Romney’s tax plan? Sure, it doesn’t add up, but he’ll fill in the details after you elect him. Romney’s spending cuts? Available just after you elect him. And then there’s the personal stuff, including his tax returns; we’re supposed to just trust him on that, too.

Greg focuses on the press’s role, but the real failure here is within the Republican Party — and the real losers from having a trust-me candidate are that candidate’s own party, too.

The way this is supposed to work is that during the nomination campaign, the party secures commitments from the winner on those issues party groups and individuals care most about. If there are major splits within the party, then the nomination battle becomes a battle over those splits, with the results either defining the party as supporting one side or the other, or perhaps a clever candidate finding a way to reconcile the two sides. If the party is united, then commitments made during the process bind the winner to the party’s agenda.

Some of that certainly happened with the Republican nomination process this time; think of Romney’s evolution on abortion, on guns and on health care. But on several broad policy areas, Romney was never really pressed, at least not publicly, and so he was able to stay ambiguous or vague.

What that means is that if he wins, Romney will be — on those issues, at least — a wild card. Sure, he may wind up sticking with the GOP party line (and there are other mechanisms Republicans have to hold him there, beginning with Republicans in Congress). But he also may veer off unpredictably. Again, that’s not going to happen on those positions on which Romney took solid positions, such as guns or abortion — but on budget matters, or for that matter, across the board on foreign policy, he just hasn’t really committed to much.

And, frankly, it’s those party actors who care the most about most of this. Well, for any particular tax provision or spending cut there are people who are directly affected, and they’ll care — but in many cases they’ll have organized representatives to fight for them at the time. The people who really care about broad policy concerns are partisans, and the ones who have the most at stake in what a presidential nominee eventually does are the ones from his or her party.

The risks to party are true, also, on the personal stuff. If Romney is concealing anything now, it’s quite likely to emerge at some point, and if he’s in the White House when it happens his presidency could be harmed or even destroyed by it. Again, this was a failure during the primaries; had his opponents been stronger (and equipped with proper opposition-research operations), Romney would probably have been forced to reveal his taxes then. Granted, there’s always the chance he could stonewall for four or even eight years, but it’s a risky start to a presidency.

None of this is to disagree with Greg’s point that the press should try to get past Romney’s dodging. But in some ways the real fault here lies squarely with the Republican Party, and they are the ones who could wind up suffering from it.