I suspect a whole lot of American voters have grown cynical and are no longer inclined to take campaign promises seriously. There’s very likely an assumption that politicians will say one thing to get elected, and then do something very different once in office.

It’s worth appreciating how mistaken these assumptions really are.

We talked last week about the new print edition of the Washington Monthly and the cover package on what Americans can expect if Republicans win the White House in November. One of my favorite pieces was written by the estimable Jonathan Bernstein who makes an important observation: what candidates say on the campaign trail is pretty much how they’ll govern if elected.

[P]residents usually try to enact the policies they advocate during the campaign. So if you want to know what Mitt Romney or the rest of the Republican crowd would do in 2013 if elected, the best way to find out is to listen to what they are saying right now.

I suspect that many Americans would be quite skeptical of the idea that elected officials, presidents included, try to keep the promises they made on the campaign trail. The presumption is that politicians are liars who say what voters want to hear to get elected and then behave very differently once in office. The press is especially prone to discount the more extreme positions candidates take in primaries on the expectation that they will “move to the center” in the general election.

But as Jonathan explains very well, political scientists know better. Presidents will fail to follow through on campaign promises, but “the main reason some pledges are not redeemed is congressional opposition, not presidential flip-flopping.”

This is no small realization. The importance comes not only from debunking a widely-held belief about politics, but also from the consequences of the insight.

There are many, for example, who’ve argued that some of the leading Republican presidential candidates — most notably, the frontrunner — may not be nearly as extreme in the Oval Office as they are on the campaign trail, so there’s less for the American mainstream to fear in the event of a GOP victory.

Bernstein’s explanation makes clear that such an argument is based on a wildly flawed premise, and voters would be wise to evaluate the candidates accordingly.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.