(AP):

Democrat Barack Obama and Republican John McCain are on the ballot in 50 states and Washington DC, Independent Ralph Nader is on 44 ballots, while Libertarian Bob Barr is on 43.

SENATE

Voters in 33 states will choose 35 senators, 33 for six-year terms. Special elections are being held in Mississippi and Wyoming, to fill the remaining four years in seats now held by gubernatorial appointees. At stake are 23 seats now held by Republicans and 12 by Democrats.

Open Republican seats are in Colorado, Idaho, Nebraska, New Mexico and Virginia.

There are no open Democratic seats. Currently, Democrats have the majority, 51-49, with two Democratic-leaning independents.

HOUSE

All 435 House seats are up for election. Republicans hold 199 seats, Democrats 235, with one vacancy.

GOVERNORS

Voters in 11 states are electing governors, with tight races in Washington, North Carolina and Indiana. Republicans are trying to chip away at the Democrats' 28-22 gubernatorial majority ahead of 2010, when 36 states elect governors. At stake is control of electoral redistricting and the support governors will lend the 2012 presidential candidates.

BALLOT MEASURES

There are 153 measures on ballots in 36 states, including divisive proposals to ban abortion in South Dakota, outlaw affirmative action in Colorado and Nebraska, and ban same-sex marriage in three states, including California - where thousands of gays and lesbians have wed since a court ruling in May.

STATE LEGISLATURES

Political control of several key state legislatures could change hands, raising the chance for one-party domination of swing states such as Pennsylvania, Nevada and New York. The party that controls the legislatures helps craft domestic policy and draw congressional districts.