My inner pessimist has been fearing lately that the conservatives have started to burn out, and that we're in danger of losing serious ground this year and in '08. Part of this is, of course, the difficulty of who the Republicans will put forward for the presidency in '08. (I've thought for a while Jeb Bush would be a great candidate, but he may well be permanently out of the running -- there's just a limit to how many president Bushes one can have.)
However, one should not overlook the ability of the Democrats to stand tall, take careful aim, and blow away their own feet. (And this from the party of gun control...) Case in point: Why exactly stage a filibuster vote that both fails and massively ticks off the radical fundraising and activist core of your own party at all the most vulnerable Democratic senators -- the more 'conservative' red state Democrats? And why did every single Democratic senator with presidential aspirations fall into this trap? "I am very moderate and stand with the American people on the issues, except that I'm to the left of half of my own party." Yeah...
It's true that this will help all the usual suspects raise money from the DailyKos and MoveOn wing of the party in the primaries. But the danger of raising money by pleasing those folks is that it just makes you look worse to the rest of the population.
Maybe the theory is that no one will remember or pay attention. It's true that this probably won't hurt them in the elections this year quite as much as impeaching Clinton (which was widely portrayed as an overreach) hurt the Republicans. However, the Republican party is fundamentally stronger than the Democratic party. It was in power and took a beating but was back up and back in power within four years. The Democrats are out of power and fighting against the tide.
The only explanation I can think of is that this is a very short term tactical move. There are congressional elections this year, and since it's not a presidential election, the assumption may well be that this is a year for energizing the base, not appealing to the wider population.