The reality is that the presidential party nearly always loses seats in the first Congressional election after their party wins the presidency.
Combine that with the extraordinarily ambitious domestic agenda Obama has been pushing (stimulus, health care reform, global warming legislation), and the fact that the country is still embroiled in a miserable recession--it could mean significant losses for the party.
The Democrats' saving grace, however, is that the Republican party these days is akin to an electoral eunuch: busy scaring the daylights out of moderates and independents with wild eyed racists like Glenn Beck and Rush Limbaugh taking center stage for the party.
Keep in mind these same moderates and independents are the ones who will ultimately swing the election.
My prediction? The Dems will still be in control of both Houses of Congress, albeit by a marginally smaller majority....