Here’s something to keep an eye on: Democrats are going to be able to exploit the narrow hold that Tea Partiers, social conservatives, and antitax absolutists have on the Republican Party — including GOP presidential nominee Mitt Romney.

Two cases in point: First, the Republicans in the Senate just successfully filibustered the Dem proposal to extend low interest rates on student loans, even though both the provision itself, and the Dem proposal to pay for it by closing tax loopholes are both popular. And second, Dick Lugar is expected to lose his seat in a primary election today — which everyone will interpret as punishment for his lack of 100% slavish devotion to extreme conservative policies.

The larger story here is very simple. Republican positions on public policy are increasingly constrained by intense but relatively small groups — making it easier for Democrats to gain political advantage on the mounting number of issues where Republican politicians can’t support popular measures.

Look at the new poll by National Journal that Greg talked about below. It tests three issues before Congress: the Violence Against Women Act renewal, the student loan bill, and the Paycheck Fairness Act:

The poll shows that Americans largely favor the Democrats’ positions on these issues: Majorities favor provisions protecting gay and lesbian victims of domestic violence and making it easier for women to sue for wage discrimination. A plurality prefers Democrats’ plans to pay for student-loan subsidies by raising taxes on some businesses, as opposed to Republicans’ plans to shift money from a preventative-health fund created as part of the 2010 health care law.

It’s worth taking a step back to realize just how astonishing it is that a major political party has managed to get itself on the wrong side of something called the Violence Against Women Act.

The truth is that Republicans are so far down the rabbit hole that they’re unable to recognize that, say, most Americans are no longer bigots when it comes to LGBT issues and so extending protections to gays and lesbians is just common sense for most people — or, to take another example, that most voters actually support raising taxes on corporations and wealthy folks.

All of which makes for a large gap between where Republicans are and where voters are that is relatively easy for Democrats to exploit — as they are doing with these three bills, and as they’ll no doubt continue to do as the campaign goes along. For Congressional Republicans, that’s going to produce votes (along with the House Republicans self-inflicted budget votes) that will be showing up in challenger attack adds all fall. For Romney, the question is whether there is any way to duck all this stuff, since he doesn’t want to alienate either marginal voters (by supporting GOP positions) or to provoke his party’s most vocal opinion leaders (by distancing himself from them). It’s going to take some pretty fancy footwork for him to manage it.