Care to elaborate?If someone makes those promises and gets elected, it hauls the whole moderate swath towards the left.
Well, it has to do with the nature of politicians. Politicians will say and do just about anything to be elected. Because of this, they are incredibly reactionary to what becomes popular, especially those politicians who are attempting to break into that large, unclaimed market - the undecided middle, that shifting, fickle sand. A great example of this in Canada is the Liberal Party. They are the centrist party, and when they look at having an opinion on any kind of issues they basically just look at which way the wind is blowing, and follow it. their position changes continually based on what the popular ideal is, and that strategy has been very successful for them. No politician can be elected without, in some way, engaging the great undecided middle. Hard left and right have already made up their minds, and their votes are already counted. So, a politician who gets elected (and has thus successfully engaged that middle) while espousing leftist ideals. Politicians looking to get elected see that success, and attempt to emulate it so that they too can engage the undecided.
The behaviour of the elected person afterwards is irrelevant to what I'm talking about, even in my original post. It's all about a shift in the broader landscape of politics, not just one person getting elected. I mean, even in the context of your political system, the president is mostly a figurehead, or someone to take the blame when shit's bad. They are probably the most visible but least important people in the day to day running of the political process. The president can say they want to do this, or that, but at the end of the day, House and the Senate hold almost all the cards. The only reason a president is important is that it means that their party holds enough seats to be a majority in the House on that election, which means that a president's promises are slightly more likely to happen.