MAD points us to this bit over at the American Prospect, which is really just a pointer to Krugman:

There are two reasons why party control is everything in this election.

The first, lesser reason is the demonstrated ability of Republican Congressional leaders to keep their members in line, even those members who cultivate a reputation as moderates or mavericks. G.O.P. politicians sometimes make a show of independence, as Senator John McCain did in seeming to stand up to President Bush on torture. But in the end, they always give the White House what it wants: after getting a lot of good press for his principled stand, Mr. McCain signed on to a torture bill that in effect gave Mr. Bush a completely free hand.

And if the Republicans retain control of Congress, even if it’s by just one seat in each house, Mr. Bush will retain that free hand. If they lose control of either house, the G.O.P. juggernaut will come to a shuddering halt.

Yet that’s the less important reason this election is all about party control. The really important reason may be summed up in two words: subpoena power…

The current Congress has shown no inclination to investigate the Bush administration. Last year The Boston Globe offered an illuminating comparison: when Bill Clinton was president, the House took 140 hours of sworn testimony into whether Mr. Clinton had used the White House Christmas list to identify possible Democratic donors. But in 2004 and 2005, a House committee took only 12 hours of testimony on the abuses at Abu Ghraib.

The official Heathen position has always been “keep either party from controlling both the White House and Congress,” but even our cynical hearts couldn’t predict how bad it would get once that happened.