Post-election polling from Rasmussen Reports shows an erosion of support for Democrats on the Generic Congressional Ballot, although they remain preferred to Republicans.

In weekly polling conducted by Rasmussen since the General Election on November 4th, an average of 42.8 percent of likely voters have pledged their support to Democratic candidates, and 39.2 percent to Republicans, a gap of 3.6 percent. By comparison, across all Rasmussen polling conducted in 2008, the gap was 45.7 to 37.0, or 8.7 percent, although the margin was slightly smaller in the months immediately preceding the election.

Rasmussen Generic Congressional BallotPeriod                   DEM      GOP     NET2007                     46.6%    35.6%   D +11.0%2008 (Pre-Election)      45.7%    37.0%   D + 8.7%__1Q 2008                43.7%    39.3%   D + 4.4%__2Q 2008                46.4%    36.3%   D +10.1%__3Q 2008                46.0%    35.6%   D +10.4%__4Q 2008 (Pre-Election) 45.6%    38.3%   D + 7.3%2008 -- Post-Election    42.8%    39.2%   D + 3.6%

It’s possible that this is a blip of some kind, or even that there’s some sort of seasonality in these numbers — the Dems’ generic ballot advantage was smaller in early 2008, for instance, before subsequently recovering to the 7-10 point range. The Occam’s Razor conclusion, however, is that Bush Fatigue was a significant part of the Republicans’ problems, and that with the election of Senator Obama, that problem is now largely behind them. Democrats no longer get to play the underdog, which is usually an enviable position in American politics.

It may also be noteworthy that the Democrats’ numbers have decreased more than the Republicans’ have increased, with more voters moving into the undecided category. There will probably be some resistance to the Republican brand for some time to come; on the other hand, Democrats may no longer be able to count on winning by default in races where they’ve nominated marginal candidates (see for instance the recent races in Georgia and Louisiana). In the event that things go poorly for the Democrats but that the Republican brand has not yet recovered — or is tarnished by party infighting — 2010-2012 could also be an interesting cycle for third-party candidates.

Barack Obama, however, remains quite popular, with a 71% approval rating from Gallup and a 67% approval rating from Rasmussen, and may well enter the White House with the highest approval ratings of any elected president since Kennedy. In both surveys, Obama is at essentially his high-water marks; there was no immediate impact from the Blagogate scandal visible in today’s set of numbers.

Nate Silver is the founder and editor in chief of FiveThirtyEight.