(CNN) -- Democrats have gained a decisive majority in the U.S. Senate, according to CNN projections, and are still aiming to acquire a filibuster-proof 60 seats.
Republican Elizabeth Dole, who lost Tuesday, said it has been a "joy" to represent North Carolina.
When Election Day dawned, Democrats needed to add nine seats to their current roster of 51 (49 Democrats and two independents allied with them) to gain the three-fifths majority that will enable them to invoke cloture, a device to end filibusters, the unlimited floor speeches by an opponent that can prevent legislation from coming up for a vote.
They will add five seats, CNN projects, bringing their total to 56.
With four races still in the balance, Democrats have an outside shot at gaining 60 seats.
In Minnesota, incumbent Republican Sen. Norm Coleman faced a strong challenge from Democrat Al Franken.
Meanwhile, in Georgia, incumbent Republican Sen. Saxby Chambliss was hanging on to a slim majority against challenger Democrat Jim Martin. If neither candidate gets more than 50 percent of the vote, the top vote-getter Tuesday will face a December runoff against the second-place finisher.
Polls closed in Alaska at 1 a.m. EST and incumbent Republican Sen. Ted Stevens appeared to be in a tight race against Democrat Mark Begich.
Stevens was convicted on seven federal corruption charges in October for filing false statements on Senate ethics forms.
In Oregon, incumbent Republican Sen. Gordon Smith and Democrat Jeff Merkley, speaker of the Oregon House of Representatives, were neck and neck.
A wild card should Democrats gain the necessary nine seats on Tuesday is Connecticut Sen. Joe Lieberman, an independent who has caucused with the Democrats but who has been an ardent supporter of Republican presidential nominee John McCain this year. If Lieberman switches his allegiance to the GOP, the Democrats are stuck at 59 seats, and the filibuster remains in play.
Two Republican incumbents were projected to lose their seats.
Democratic Gov. Jeanne Shaheen will defeat Republican first-term incumbent John Sununu in a rematch for the Senate seat in New Hampshire, CNN projects. In 2002, Sununu defeated Shaheen in a midterm election.
In North Carolina, another first-term incumbent, Republican Sen. Elizabeth Dole will lose to Democratic state Sen. Kay Hagan, CNN projects.
"What a difference a year makes," Hagan told a cheering crowd Tuesday night. "A little over a year ago, when I got into this race, the press, the pundits, other politicians and all, were ready to write this race off and hand Elizabeth Dole the keys to her office for another six years.
"But it's not her office," Hagan said, receiving thunderous applause. "And we need better." Watch Hagan have the last word to her critics »
Dole, a first-term incumbent, had been considered a safe seat for Republicans early in the election cycle, but she was targeted heavily by national Democratic Party ads.
Dole told a crowd that is has been her "joy" to represent North Carolina. "I have done my level best to make you proud," she said. Watch Dole thank her supporters »
Other results, according to CNN projections:
A filibuster is a senator's ability to speak without a time limit on the Senate floor, preventing any votes on measures before the body.
A cloture vote of 60 - three-fifths of the Senate - is needed to end debate and stop a filibuster.
Cloture was instituted in 1917 at the behest of President Woodrow Wilson, who became frustrated when filibusters blocked his attempts to get the United States involved in World War I.
Cloture originally required a two-thirds vote of the Senate and was amended to the current three-fifths vote in 1975.
Democrat Dick Durbin will defeat Republican Steve Sauerberg for the Senate race in Illinois; former Gov. Mark Warner, a Democrat, will win the U.S. Senate seat in Virginia. CNN also projects that Democratic Rep. Mark Udall will win the Senate seat in Colorado left open by retiring Republican Sen. Wayne Allard. Udall will defeat Republican Bob Schaffer, a former three-term congressman, in the fifth Democratic upset of the night.
Democratic incumbent John Kerry, the 2004 presidential candidate, will defeat Republican challenger Jeff Beatty for the Senate race in Massachusetts.
Democratic incumbent Joe Biden will defeat Christine O'Donnell for the Senate race in Delaware, but he will have to give up his seat because he is now vice president-elect.
Biden's successor will be named by Delaware's Democratic governor, Ruth Ann Minner.
More Democratic wins will go to incumbent Sen. Jay Rockefeller, who will take the U.S. Senate seat in West Virginia, and senatorial incumbent Frank Lautenberg, who is expected to defeat Republican Dick Zimmer in New Jersey, CNN projects.
Additional CNN projections include two-term Democrat Tim Johnson, who recovered from a brain hemorrhage he suffered in 2006, will defeat Republican Joel Dykstra for the Senate seat in South Dakota. Michigan's senior senator, Democrat Carl Levin, will win a sixth term, defeating Republican state Sen. Jack Hoogendyk.
CNN projects that Democratic Sen. Jack Reed will win a third term in Rhode Island, defeating Republican Robert Tingle, who also ran against Reed in 2002. CNN also projects four-term incumbent Democratic Sen. Tom Harkin will defeat Republican Christopher Reed in the Senate race in Iowa.
In Montana, Democratic incumbent Max Baucus will win a sixth term in Montana's Senate race, defeating attorney Bob Kelleher, a Republican, according to CNN projections. Also, one-term Democratic incumbent Mark Pryor will defeat Green Party challenger Rebekah Kennedy for the Senate race in Arkansas.
Under Illinois law, if a U.S. Senate seat becomes vacant, the governor appoints a replacement to fill the vacancy. That appointed senator serves until the next scheduled federal election year, in this case, 2010.
Obama was elected in 2004 and would have been up for re-election in 2010 anyway, thus the appointed senator would serve for the rest of Obama's term. The governor of Illinois is Rod Blagojevich, a Democrat.
CNN projects these Republican wins:
Incumbent Mitch McConnell, the Senate minority leader, will win the Senate race in Kentucky, defeating Democratic businessman Bruce Lunsford. "The most exhilarating feeling in life is to be shot at and missed..." McConnell said, quoting Winston Churchill. Watch McConnell talk about his victory »
Republican Lindsey Graham, also an incumbent, will win the Senate race in South Carolina. Two-term Republican incumbent Susan Collins will defeat Democratic challenger Tom Allen for the Senate seat in Maine, Republican Lamar Alexander will defeat Democrat Bob Tuke for the Senate seat in Tennessee, and Republican incumbent James Inhofe will defeat Democrat Andrew Rice for the Senate seat in Oklahoma.
CNN projects that Republican incumbent Mike Enzi will win the Senate race in Wyoming, defeating Democrat Christopher Rothfuss, and Republican incumbent John Barrasso will win the Senate race in Wyoming, defeating Democrat Nick Carter. Barrasso was appointed to the seat in June 2007 after Sen. Craig Thomas died of leukemia.
CNN projects that Republican incumbent Pat Roberts will win a third term in the Senate race in Kansas, defeating Democrat Jim Slatter, a former six-term congressman.
In Alabama, Republican Jeff Sessions will defeat Democrat Vivian Davis Figures for the Senate race, and Republican incumbent Thad Cochran will win a sixth term in Mississippi's Senate race, defeating Democrat Erik Fleming, a former state lawmaker, CNN projects.
CNN projects that Republican incumbent John Cornyn will clinch a second term in the Texas Senate race, defeating Democratic state Rep. Rick Noriega.
Of the 35 Senate seats on the line this year, 23 were held by Republicans. Five Republican senators are retiring: Pete Domenici of New Mexico, Wayne Allard of Colorado, John Warner of Virginia, Larry Craig of Idaho and Chuck Hagel of Nebraska.
CNN projects that Democrat Tom Udall will defeat Republican Steve Pearce for the seat Domenici held and that former Gov. Mike Johanns, a Republican, will win the Senate seat in Nebraska, replacing Hagel. Johanns, who served as secretary of agriculture in the Bush administration until 2007, will defeat rancher Scott Kleeb, a Democrat.
Republican Lt. Gov. Jim Risch will defeat Democratic former Rep. Larry LaRocco for the Senate race in Idaho. Risch replaces Craig, whose arrest in a police sting operation in a Minneapolis, Minnesota, airport bathroom made his Senate post one of the most closely watched in 2007.