Congressional Democrats with an eye on political history should be nervous. California GOP chairman Ron Nehring highlights an uncomfortable reality for them as they head into 2010 with the least popular president at this stage in his presidency in decades:

The magnitude of the net losses suffered by the President’s party in Congress has been in direct, inverse proportion to the President’s public approval rating on Election Day. The party in control of the White House suffered the most in 1966, 1974 and 1994 when the incumbent’s approval ratings were all under 50%. High approval ratings of President Clinton in 1998 (66%) and President Bush in 2002 (63%) helped the governing party gain seats in those two years — a historical aberration.

How bad could it be? Bad. It seems that “an approval rating under 50% has typically resulted in a wipeout of 41 House and 477 state legislative seats lost by the President’s party. Barack Obama’s approval rating within the last week has hovered between 47% and 50% in Gallup’s surveys.”

We already saw Bob McDonnell leverage unpopular Obama policies (which have gotten even less popular in the past month) to win a state race. While avoiding running against Obama personally, McDonnell certainly ran against the Obama agenda (cap-and-trade, card check, ObamaCare, higher taxes). And it worked — spectacularly so. Those Congressional Democrats up in 2010 will have an even tougher time, for nearly all of them voted at one procedural stage or another for a series of hugely unpopular or failed measures. There was the $787 stimulus plan, the huge spending bills, the House bills on PelosiCare and cap-and-trade, and the funding of the Obama gambits to move terrorists to the U.S. and to give KSM his civilian trial. It’s not as if they’ve been innocent bystanders of the leftward lurch that has inflamed the electorate.

So it isn’t simply that Obama won’t be able to give lawmakers much cover if his approval ratings don’t pep up; it’s that the lawmakers are tied at the hip to the very agenda items that have helped make Obama so unpopular. Now, Obama’s poll numbers can change, the economy can bounce back, and the White House and Congress can embark on a major course correction. Those Democrats on the 2010 ballot better hope for all three.