How many senators are up for re-election in 2008?
In total, 35 U.S. Senate seats are at stake on Nov. 4. Of these, 23 are currently held by Republicans, while the Democrats are defending 12 slots. Five of the Republican seats are open after a chain of GOP retirements including Trent Lott in Mississippi, Chuck Hagel in Nebraska, Pete Domenici in New Mexico, Larry Craig in Idaho and Wayne Allard in Colorado.
 Two U.S. Senate seats are up for election in Wyoming and Mississippi because of special elections. In Wyoming, Republican John Barrasso was appointed to finish Sen. Craig Thomas' term after his death in 2007 and is expected to win the special election. In Mississippi, Republican Roger Wicker is fighting to keep the Senate seat he filled after Sen. Trent Lott retired. Winners of those seats will serve a partial term that lasts until 2013.
The remaining Senate seats -- 39 Democratic and 26 Republican -- are not up for election this fall.
What is the current balance of power in the Senate?
After the 2006 elections, Democrats gained a slim 51-49 majority in the Senate when Sens. Joe Lieberman of Connecticut and Bernie Sanders of Vermont, both Independents who caucus with the Democrats, are included in the count.
Lieberman, who was the Democratic vice presidential nominee in 2000, became an Independent in 2006 after losing the Democratic primary. He went on to win re-election as an Independent and supports Republican Sen. John McCain for president. If he chose to caucus with the Republicans, it would mean one less seat for the Democrats to count on.
What is a filibuster-proof majority?
Democrats are trying to reach 60 seats in the Senate, which will give them a filibuster-proof majority. In the Senate, 41 members can block legislation with a filibuster, which creates a legislative roadblock by extending debate on a bill rather than putting it up for a vote. Both parties have employed the tactic over the course of Congressional history.
Because the Democrats are expected to maintain or build on their majority in the House of Representatives, a 60-seat majority will make much it easier to pass bills. If Democratic candidate Sen. Barack Obama is elected president, legislation passed by the Democratic-controlled Congress will also face less of a chance of being vetoed.
With 60 seats, the Senate can also override a presidential veto.
Which Senate seats could change hands?
Democrats are running competitive races to nab Republican-held Senate seats in Oregon, Alaska, Colorado, New Mexico, Minnesota, Virginia, North Carolina and New Hampshire. While Louisiana looked like an opportunity for Republicans to pick up a Democratic seat by challenging incumbent Sen. Mary Landrieu, the race appears less competitive in the final weeks of the campaign.
Editor's note: An earlier version of this report incorrectly stated that the Republicans used 104 filibusters in the 110th congress. The story has since been updated.
-- By Anna Shoup, Online NewsHour
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