Senators run every four years. The 2012 election is likely to undo the Democratic sweep of the Senate that took place in 2006. During this sweep, many traditionally Republican states went Democratic.
In 2012, 21 Democratic seats are up for election, along with 10 safe Republican seats. It is likely that Republicans will take the Senate.
The Congress controls the budget.
The last time we had a balanced budget happened during the Clinton Adminstration when we had a Democratic President and Republican Congress.
If Republicans take the Senate then we will have a lame duck president standing against a Conservative Congress wishing to earn its free market credentials by cutting government spending.
In contrast, if Romney wins, we will have a Progressive Republican president seeking to court favor with the people by expanding government spending. We saw this combination under George W. Bush who increased government spending through a dramatic rise in deficit spending.
If the driving issue for your vote is deficit spending, I believe the best option for the 2012 election is to vote for a Republican Senator and for a Third Party.
If there is a Split Vote and Obama wins a lame duck term without winning 50% of the vote, then the split vote will force Obama to move to the center.