Outside of changing course in Iraq, I think it's fairly obvious that there really isn't one — especially when it comes to domestic policy. And here's a pretty good reason to be skeptical about any claims made on the basis of the Democrats' popular-vote margin in the Senate: Almost all of it came from safe seats in California, New York and Massachusetts.
This is true—Dianne Feinstein, Hillary Clinton, and Ted Kennedy won by landslides totalling over 4 million votes, and the Democrats got 6.6 million more Senate votes than Republicans. But you can spot the Democrats another million votes in Indiana. They didn't challenge iconic Sen. Richard Lugar, and he racked up 1.16 million votes against a Libertarian (hooray) who scored 170,000. Spot them another million votes in Texas, where Kay Bailey Hutchison mutilated a hapless Democratic lawyer by a 1.1 million vote margin. The Democrats nominated placeholders who got creamed in Mississippi, Wyoming, and Utah, but those states are so vote-poor that they basically cancel out the Democratic landslides in North Dakota, Nebraska and West Virginia. So split the difference and the Democrats came out of the election with around 4.5 million more votes than Republicans in competitive races. Compared to George W. Bush's epochal 3 million vote margin in 2004, not too shabby for Chuck Schumer and crew.
Of course the better gauge of a national party's strength is how they fared in the 435 House districts, and Democrats won there by between 7 and 10 points, depending on how you count uncontested races. But isn't this moot anyway? When the Republican president proclaims it a "thumpin'," let's call it a thumpin' and move on.