Politicians change their mind and break promises all the time. But on foreign policy, their campaign-season talk is less reliable than on other issues — for better or for worse.

Woodrow Wilson ran for reelection on the motto, “He Kept Us Out of War.” In his second term, he didn’t keep us out of war.

George W. Bush ran for president opposing nation-building and calling for a “humble foreign policy.” Yeah. How’d that turn out?

And President Obama sure didn’t promise to keep open Guantanamo, build a kill list to target with his drones, or launch a new — and illegal — war in the Arab world.

So, whatever Romney says tonight, I wouldn’t count on it being reflected in his foreign policy if he becomes President. Phil Klein suggested as much in a blog post today, saying “Romney is likely to pursue a more restrained approach than Bush because he’s more risk averse.”

One reason: almost every presidential candidate has “book learned” foreign policy. Once they get in the White House, they get many new advisors — most of whom know the subject much more than they do — and they get classified information. However strong your ideological convictions, some top-secret intercepts can sway you.

Eli Lake, a great national security reporter, has known this for a while. He wrote in 2008 that Obama would be less Jimmy Carter on foreign policy, and more Ronald Reagan:

You can easily see how this will play out. Obama will enter office with a set of somewhat inchoate instincts about American power and the importance of outsourcing force. These instincts will mesh with the evolving thinking of his top commanders, who have also begun to realize the limitations of an overstretched army and the value of counter-insurgency. And that brings us back to the situation room on Obama’s first day. If he and Petraeus can overcome whatever awkwardness lingers, they will discover a mind meld and an emerging doctrine– a doctrine that looks a lot more like Ronald Reagan than Jimmy Carter.

I expect Romney to be more hawkish than Obama tonight — but that doesn’t mean he would be more hawkish as a President.