Tonight, after the polls have closed and votes have been tallied, Americans will have elected the next president of the United States. Regardless of whether the victory goes to President Obama or Mitt Romney, however, the next president's ability to govern will hinge on which party holds a majority in the House and the Senate.

In order to overtake the slim Democratic majority in the Senate, Republicans need to pick up at least four seats tonight. A year ago, that seemed like a pretty reasonable goal for the GOP. But in the final days of the campaign season, amid a handful of unforced errors by Republican Senate candidates, it's looked increasingly out of reach. Democrats, meanwhile, seem equally unlikely to pick up the 25 seats they'd need to regain control of the House.

Below, CBS News takes a look at the top House and Senate races to watch as polls close across the country.

3:00 p.m. AKST

Indiana Senate: The Indiana Senate race has been full of surprises for Republicans: First, conservative Richard Mourdock defeated longtime moderate incumbent Dick Lugar in primary; more recently, after Mourdock suggested that pregnancies resulting from rapes were "something that God intended to happen," an already-close race tilted in conservative Democrat Joe Donnelly's direction. A Donnelly win there tonight would make it even tougher for Republicans to overtake Democrats in the chamber.

Virginia Senate: In Virginia, voters face a familiar pair of candidates when making their choice for the Senate: Tim Kaine, the Democratic former governor, is running against Republican George Allen, a former senator and governor, for the state's open seat. The contest has been among the most closely-watched in the country, particularly given Virginia's status as a critical swing state in the presidential election, and was neck-and-neck for months. In recent weeks, however, Kaine appears to have eked out a slim lead.

3:30 p.m. AKST

Ohio Senate: Democratic Sen. Sherrod Brown started his re-election campaign with a solid lead over his competitor, Ohio Treasurer Josh Mandel. But amid a recent onslaught of outside money, his lead appears to have winnowed down to just a few points, giving Mandel some cause for hope. If he's successful in unseating Brown, Mandel would be handing the GOP a much-needed pick-up: Even if Republicans can win seats in all the states where Republican Senate candidates are either tied or have the lead, they'll still be one Senator short of a tie with Democrats without a victory in Ohio, according to the New York Times.

Ohio - U.S. House, 16th District: GOP Rep Jim Renacci v. Democratic Rep. Betty Sutton: Because Ohio lost two congressional seats after the 2010 census, Renacci and Sutton are forced to run against each other. The bitterly fought campaign between the two candidates has also attracted millions of dollars from outside groups who are waging a vocal war in the race.

North Carolina - U.S. House, 7th District : Democratic Rep. Mike McIntyre v. David Rouzer: Incumbent Rep. Mike McIntyre is one of the endangered conservative Blue Dog Democrats who represent a district that leans Republican. His newly mapped district includes suburbs of Raleigh and areas along the coast. His challenger, state senator David Rouzer has received the backing of outside groups in a race that Republicans hope help dash Democrats' hopes of picking up seats. But because he survived the Republican landslide in 2010, when the GOP picked up more than 60 House seats, McIntyre is a viable candidate.