Come January, the House will be composed of an energized conservative Republican majority and a Democratic minority that has become more liberal. At the same time, a more closely divided Senate could make it harder to assemble the 60 votes needed to pass most bills.
That could be a recipe for legislative gridlock.
In the past year, Senate Majority Leader Harry Reid (D., Nev.) has struggled to pass legislation even with a 59-seat majority, thwarted by Republican filibusters. The election's outcome remained uncertain in Alaska and Washington state, but it's clear Democrats will emerge with a far smaller majority of 52 or 53 seats out of 100.
In such a narrowly divided Senate, a bipartisan coalition will be required to tackle any difficult issue. Yet some Democrats leaving the Senate, like Arkansas's Blanche Lincoln and Indiana's Evan Bayh, are among the most open to working with Republicans.
Reid said the election results left the parties no choice but to work together—and that he wanted to do so. But he also said Republicans have been uniformly obstructionist over the past two years.
"The ball is in their court," Reid. "We made the message very clear that we want to work with the Republicans. If they're unwilling to work with us, there's not a thing we can do about it."
GOP leaders don't see it that way, saying voters rewarded them for blocking Democratic initiatives.
"What the American people were saying yesterday is they appreciate us saying no to things the American people indicated they were not in favor of," said Senate Minority Leader Mitch McConnell (R., Ky.).
Now, he said, it's up to the Democrats to move the Republicans' way.
That might be easier said than done, with the new Senate still including a strong faction of impassioned liberals, like Sen. Bernie Sanders (I., Vt.), as well as a group of staunch conservatives like Sen.-elect Rand Paul (R., Ky.).