Gallup's generic congressional ballot provides a summary measure of current voting intentions for Congress. This currently suggests the 2010 midterm elections could be highly competitive, and possibly a strong Republican year if usual turnout patterns prevail.
Although Democrats retain a significant advantage in party affiliation, that advantage has dwindled over the course of this year. Also, there are ominous signs for the majority party in terms of near-record-low congressional job approval and continuing low national satisfaction ratings.
The Democrats will contest the 2010 elections with their fellow partisan, Barack Obama, in the White House. Right now, Obama's approval ratings are middling, in the low 50s, suggesting he would not be able to minimize Democratic losses to a great degree if the elections were held today. Further erosion of Obama's popularity between now and next November could prove damaging to the Democratic Party. First-term presidents who had sub-50% approval ratings at the midterms -- including Harry Truman, Lyndon Johnson, Jimmy Carter, Ronald Reagan, and Bill Clinton -- saw their parties suffer large congressional seat losses. In contrast, a recovery in Obama's approval rating -- particularly to above 60% -- could limit Democratic losses. (continues here at Gallup)
Obama's job approval at 50% in Gallup polls.