In the House, there’s often a strong correlation between the two, although it often depends more on how the President’s Party did two years ago. In the Senate, the incumbent party and their President’s popularity often does impact Senate gains and losses, just as they did in 2006 and 2008, and figure to do it 2010.
But if we look deeper we see the Democrats’ biggest Senate gain since 1958 was 8 seats in 1986, a year they only took 5 seats in the House. In 2000 they picked up 5 seats, despite losing the Presidency and winning only one seat in the House.
What gives? To look at Senate seats you have to actually separate them into three different groupings, Class I, Class II, and Class III senators.
Let’s start with Class III. In 1980, Democrats had 24 seats up for election, while the Republicans had only 10. In such cases the Republican 10 are those that they didn’t lose in the 1974 election and be 10 of the safest seats. Regardless of the Presidential election those numbers alone made Democratic losses fairly certain. Ronald Reagan won overwhelmingly. The Democrats and Republicans split the 24 Democratic seats 12-12, while the Republicans retained their 10 seats.
That takes us to 1986. The Republicans now had to defend 22 seats, while the Democrats needed to defend 12. The GOP was holding too many seats in Democratic skewing states and went 13-9. The Democrats went 11-1 with their seats. This brought the seat distribution back to a nice equilibrium. In 1992 the Democrats went 19-2 in their seats. The GOP went 14-2 in theirs.
In 1994, Class I seats were up for election. Due to resignations and a death the Democrats had 22 seats up, while the Republicans had only 12. It was a strong Republican year. The Democrats won 14 of their own seats and lost 8, while the Republicans retained all of their seats. If the number of seats up had been reversed, the Democrats would’ve had little or no losses.
Despite a Bush victory and no House movement, the Democrats took 4 Republican seats in 2000. The Democrats went 13-2, while the Republicans went 13-6. The two sides a relatively equal number of seats up in 2006, so the Democratic momentum gave them 6 additional seats.
In 2012, the Democrats could have as many as 23 seats up for grabs, if we count the independents who caucus with them and they win the elections in New York and West Virginia as expected. Only one of the 10 Republicans seats, Massachusetts, is in serious play, while two others, Nevada and Arizona, could be. Even if Obama is popular and wins in a landslide, the Democrats will likely have losses. The Republicans lost 2 seats in 1972 and 1984 despite Nixon and Reagan landslide re-elections.