For the last two years, the Democrats have held a very slim margin in the United States Senate -- and the 51-49 lead comes from two independent senators who caucus with Democrats.

Interactive: Balance Of Power

This year, with 35 Senate seats up for elections, the Democrats seem poised to gain on their advantage -- possibly getting to the 60 seats needed to stop a filibuster.

Twenty-three of the seats up for grabs this year are held by Republicans, while Democrats control 12. Two races this year -- one of the seats in Mississippi and one of the seats in Wyoming -- are special elections to fill the remaining four years of terms that started in 2006.

Sen. Roger Wicker of Mississippi holds a one-point lead over Ronnie Musgrove in polling for the special election seat. Mississippi's other seat -- held by Sen. Thad Cochran -- and both Wyoming seats appear to be safely Republican, polling indicates.

There are five retirements from the Senate prior to this election -- all from seats held by Republicans: Wayne Allard, Colo.; Larry Craig, Idaho; Chuck Hagel, Nebraska; Pete Domenici, N.M.; and John Warner, Va.

According to polling, Democrats hold double-digit leads in Colorado, New Mexico and Virginia.

2008 Senate Races
Democratic Incumbents
Senator	State
Mark Pryor	Arkansas
Joe Biden	Delaware
Dick Durbin	Illinois
Tom Harkin	Iowa
Mary Landrieu	Louisiana
John Kerry	Massachusetts
Carl Levin	Michigan
Max Baucus	Montana
Frank Lautenberg	New Jersey
Jack Reed	Rhode Island
Tim Johnson	South Dakota
Jay Rockefeller	West Virginia
Republican Incumbents
Jeff Sessions	Alabama
Ted Stevens	Alaska
Saxby Chambliss	Georgia
Pat Roberts	Kansas
Mitch McConnell	Kentucky
Susan Collins	Maine
Norm Coleman	Minnesota
Thad Cochran	Mississippi
Roger Wicker	Mississippi
John Sununu	New Hampshire
Elizabeth Dole	North Carolina
Jim Inhofe	Oklahoma
Gordon Smith	Oregon
Lindsey Graham	South Carolina
Lamar Alexander	Tennessee
John Cornyn	Texas
Michael Enzi	Wyoming
John Barrasso	Wyoming

There are several races where the Republican incumbent will pushed to the wire to hang on to their seats, polling shows.

Alaska: Sen. Ted Stevens vs. Mark Begich

Stevens has served Alaska in the Senate since his appointment in 1968 and is the longest serving Republican senator.

This year, he is being challenged by Begich, the mayor of Anchorage. According to an Ivan Moore poll that ended Oct. 19, Begich holds a 46-45 lead.

Stevens has been dogged by a corruption trial for failing to properly report gifts. A jury found him guilty on Oct. 27, eight days before the election. The charges related to renovations to his home and alleged gifts from VECO Corporation, claimed to be worth more than $250,000.

Since his first election to a full term in 1972, Stevens has never received less than 66 percent of the vote, and is currently the fourth-longest serving member of the Senate, after Democrats Robert Byrd, Ted Kennedy and Daniel Inouye.

Georgia: Sen. Saxby Chambliss vs. Jim Martin

Chambliss, who defeated incumbent Max Cleland in a contentious race in 2002, held a 29-point lead in some polls earlier this summer. In an Oct. 22 Rasmussen poll, the lead was down to 2.

Until a few weeks ago, Chambliss was thought be a safe bet for re-election, but with the race tightening, national Democrats have begun pouring money into the state supporting Martin.

Cleland, who had three limbs amputated after a Vietnam grenade blast, lost to Chambliss that year after serving one term.

"Saxby Chambliss won his Senate seat in the final days by putting my picture next to Osama bin Laden and lying about me. It was despicable, but it worked," Cleland wrote in the fundraising letter. "This year, we can't let Chambliss use the same vile tactics to defeat Democratic challenger Jim Martin."

Chambliss' 2002 ad still elicits a visceral reaction from Democrats, who call it an egregious example of Republican manipulation of terrorism fears. It showed photos of bin Laden and Saddam Hussein while accusing Cleland of being soft on defense for blocking the creation of a homeland security department. Cleland supported the concept but had opposed some legislative proposals because he wanted more labor protections for employees.

Chambliss' campaign had no immediate comment to Cleland's letter.

Despite what may happen in November, the drama may stretch out into December, when a run-off election would be held if the winning candidate does not get 50 percent of the vote.

Kentucky: Sen. Mitch McConnell vs. Bruce Lunsford

Due to McConnell's ties to President George W. Bush, Democrats have made defeating McConnell -- the Senate minority leader -- one of their top priorities.

McConnell has been touting his role as his party's leader in the Senate. He said Kentucky voters would be making a bad trade if they replaced him with Lunsford in the election.

Lunsford said that having the title "leader" does not make one. Lunsford said it is the time to get leadership out of fresh ideas.

Minnesota: Sen. Norm Coleman vs. Al Franken (D) vs. Dean Barkley (I)

In the third-party spirit of former Gov. Jesse Ventura, Minnesota's Senate race is a three-way tussle.

Barkley, who was Ventura's campaign manager during his upset victory 10 years ago, has been enjoying support against the incumbent Coleman -- in his first re-election bid -- and writer/comedian Al Franken.

The latest polling in the race -- a Rasmussen poll ending Oct. 22, has Franken with a 41-37-17 lead. Coleman had a 10-point lead at the start of October, with Barkley's support hitting a high point of 19 percent.

Coleman won his seat in 2002. He defeated former Vice President Walter Mondale, who was running after Sen. Paul Wellstone was killed in a plane crash on his way to a campaign event.

The New York Times reported that Sen. John Ensign, the chair of the National Republican Senatorial Committee, said earlier this year that "we're very grateful to have Al Franken" challenging Coleman.

New Hampshire: Sen. John Sununu vs. Jeanne Shaheen

Shaheen, a former New Hampshire governor, has been consistently ahead of Sununu -- the Senate's youngest member -- in a rematch of 2002 when Shaheen lost by 4 points. The average of the most recent polls has Shaheen holding a 6-point lead, and will have former President Bill Clinton campaigning on her behalf the days prior to the election.

The seat was seen as winnable for the Democrats after the 2006 elections, when Democratic Gov. John Lynch was re-elected with the largest margin in state gubernatiorial history, Democrats knocked off both sitting Republican House members, and Democrats won power of both houses of the state legislature for the first time since 1874.

Sununu is the son of the state's former governor, John Sununu, who was the chief of staff for President George H.W. Bush.

North Carolina: Sen. Elizabeth Dole vs. Kay Hagan

Dole, the wife of 1996 Republican presidential candidate Bob Dole, is statistically deadlocked with Hagan, a state senator. (Watch: NC Senate Race Heats Up)

Hagan holds leads within the margin of error in both the WSOC-TV poll and the Civitas Institute polls. Dole has taken some heat because she was the head of the National Republican Senatorial Committee in 2006, which lost control of the Senate to the Democrats.

WXII-TV in Winston-Salem said that the Civitas poll showed that Hagan leads Dole 41-38 among those who initially voiced support for a candidate, but when undecided voters are asked which way they lean, Hagan’s lead becomes 44-41.

"Our polling shows Kay Hagan has sustained the slim lead we first showed in our polling two weeks ago of just 3 points over Elizabeth Dole," Francis De Luca, executive director of the Civitas Institute, told WXII-TV. "The volume and tone of attack ads in this race continues to increase and our poll shows more voters shifting to undecided."

The race has drawn significant attention from outside interest groups, which has poured millions of dollars into the race. (Watch: NC Senate Race Gaining National Attention)

According to the Campaign Media Analysis Group, which tracks campaign advertising, at least 11 groups have bought TV ads that total about $10 million. The candidates account for an additional $7 million.

Oregon: Sen. Gordon Smith vs. Jeff Merkley

Portland television station KPTV reported that the race between Smith and Merkley, the speaker of the state House of Representatives, has become the most expensive in the nation this year with each side spending more than $20 million. (Watch: Hard-Fought Oregon Senate Race Nears End)

Smith has tried to campaign as a moderate Republican, holding press conferences with scores of Democrats behind him, and has touted his work in the Senate with Democratic presidential candidate Barack Obama in his advertising.

Merkley has attacked back, telling KPTV that he has voted against Democratic Sen. Ron Wyden, Oregon's other senator in Washington, more than 1,700 times.

Depending on the poll, the race ranges between a tie to a 7-point Merkley lead.