It's been obvious for years that the Republican Party is massively overly responsive to its most conservative constituents, who are also usually its loudest. Ultra-conservatives don't actually make up a majority of the Republican electorate, but Republicans govern like they do. It's true that congressional Republicans are beholden to constituencies more conservative than America as a whole, but elected Republicans apparently think their constituencies are even more conservative than they actually are. This poll helps explain why the party keeps overreaching, imagining it has a mandate to impose radical and unpopular right-wing policies whenever candidates win low-turnout midterms, and even when they actually lose elections but remain in power thanks to quirks of our political system. Hence, the Gingrich shutdown, the Clinton impeachment, George W. Bush's Social Security "reform" push, and just about everything congressional Republicans have done since January 2009.
Liberal politicians, meanwhile, don't imagine that their constituents are super-liberal. A majority of them also believe that their constituents are more conservative than they actually are. Which, well, that explains your Democratic Party since the Clinton administration. They weren't polled, but I'm pretty sure "nonpartisan" political elites in the media share the exact same misperception. ("It's a center-right country," we hear all the time, which it turns out is both meaningless and untrue.)
