Although there will be nearly 500 results coming in from around the country, the contest for control of Congress boils down to a few dozen competitive races.
House of Representatives
Each state is represented in the house by at least one representative who is elected for a two-year term. The number of representatives a state sends to the house is based on its population.
All 435 seats in the house are up for grabs. The Democrats need to gain 15 seats currently held by Republican representatives to take control for the first time since 1994.
The first indication of trouble for the Republicans could come when polls close at 7pm (0000 GMT) in Indiana and Kentucky. Democrats have fielded moderate candidates to challenge Republican incumbents in several districts.
Should Democrats win there, it is conceivable that later in the evening they will also win seats in the liberal-leaning northeast that are held by moderate Republicans.
House Republicans Nancy Johnson, Christopher Shays and Rob Simmons in Connecticut, as well as Mike Fitzpatrick, Curt Weldon, Jim Gerlach, Don Sherwood and Melissa Hart in Pennsylvania could be under threat.
Each state is represented by two senators who serve six-year terms. One-third of the seats in the Senate are up for grabs at every election.
Thirty-three seats in the 100-member assembly are being voted on. Seventeen of them are currently held by Democrats, 15 by Republicans and one by a retiring independent.
Election authorities are carrying
The Democrats need to gain six seats to take control for the first time since 2002.
The earliest indication of a Senate trend may be seen in Virginia, where polls close at 7pm (0000 GMT). Republican incumbent George Allen faces a tough challenge from Republican-turned-Democrat James Webb, an author and former navy secretary in the administration of Ronald Reagan, a former president.
Lincoln Chafee, a Republican senator for Rhode Island, and Bob Menendez, a Democratic senator in New Jersey, are also in tight races whose outcome could say plenty about the overall results.
Democrats believe they can win Pennsylvania, Tennessee and Ohio, three states with incumbent Republican senators.
The governor is the chief executive of the state and serves a four-year term, except in New Hampshire and Vermont, where they are in power for two years.
Thirty-six of the 50 state governorships are being voted on, with high-profile contests taking place in California, New York, Massachusetts and Ohio.
The Democrats need to gain only four states to tip the balance of power their way. The Republicans currently have 28 governors, the Democrats 22.
Governors’ offices do not affect Congress but are regarded as important weapons for presidential candidates, helping to raise funds and delivering votes at election time.