ATLANTA, Georgia (CNN) -- Sen. Joseph Lieberman conceded to anti-war cable executive Ned Lamont in the Democratic primary for the Connecticut Senate nomination Tuesday night but vowed to run as an "independent Democrat" this fall.
Lamont carried 52 percent of the vote to 48 percent for Lieberman, taking victory on a tide of opposition to the Iraq war and what his supporters said was Lieberman's unwavering support for it.
Lieberman, who was former Vice President Al Gore's running mate in 2000, is seeking a fourth Senate term. Lamont, a former Greenwich city councilman, is running his first statewide campaign. (Watch Lieberman reveal his plans after the primary loss -- 2:20)
Lieberman told his supporters he had congratulated Lamont, but added: "We've just finished the first half and the Lamont team is ahead, but, in the second half, our team -- Team Connecticut -- is going to surge forward to victory in November."
Lamont, speaking to his supporters at his victory celebration, urged Lieberman not to run.
"I want to thank Sen. Lieberman for his campaign," Lamont said. "I want to thank him for the dignity and decency in which he has represented our state and our country, for many, many years.
"I'm hoping that over the next few days that he will come to the conclusion that the party is going to stick together and go forward united," Lamont said.
Also on Tuesday night, Georgia Rep. Cynthia McKinney lost in a runoff to Democratic challenger Hank Johnson.
And in Missouri Claire McCaskill, as expected, won the Democratic nomination for the U.S. Senate. She will face Republican Sen. Jim Talent this fall, according to The Associated Press.
Primaries are also taking place in Colorado and Michigan, but the Democratic race in Connecticut was the most closely watched of Tuesday's races.
Lieberman faults 'polarization'
Lieberman said he was disappointed by the results but more disappointed in how the primary campaign was conducted.
"The old politics of partisan polarization won today. For the sake of our state, our country and my party, I cannot, I will not let this result stand."
Lieberman had for some time threatened to run as an independent in November if he failed to claim the party's nomination, further enraging party activists and many Web commentators, who have boosted Lamont.
A source close to Lieberman told CNN that Lieberman has the 7,500 signatures necessary to get on the ballot as an independent candidate.
Voter turnout was high in the high-profile primary fight, according to Connecticut Secretary of State Susan Bysiewicz. Bysiewicz predicted Tuesday that turnout for the primary could reach 45 percent to 50 percent, nearly double the normal turnout for a primary election.
Both Lieberman's and Lamont's campaigns claimed that high voter turnout would help their chances for victory.
In a state President Bush lost by a 10-point spread in 2004, Lieberman's critics have crystallized their complaint in a single image: Bush's embrace of Lieberman after the 2005 State of the Union address, which Lamont fans dubbed "The Kiss."
Lieberman called accusations that he has been a "cheerleader" for Bush "ridiculous." Former President Bill Clinton, former Georgia Sen. Max Cleland and fellow Connecticut Sen. Chris Dodd campaigned for him in recent weeks.
"I have opposed most of what this president has asked us to do in Congress," Lieberman said. "And secondly, that somehow I am not a real Democrat? It is outrageous."
On Tuesday afternoon, Lieberman's campaign accused Lamont's of "dirty politics" after a denial-of-service attack knocked out the Lieberman campaign Web site. Lieberman spokesman Sean Smith compared the hack to the kind of tactics used by Karl Rove, Bush's top political adviser. (Full Story)
"There is no place for these Rovian tactics in Democratic politics, and we demand that our opponent calls off his supporters and their online attack dogs," Smith said.
When asked by reporters at a campaign stop Tuesday if he or his campaign was responsible for the incident, Lamont said, "No, it's just another scurrilous charge."
Liz Dupont-Diehl, a Lamont spokeswoman, told CNN that the campaign "denounced and condemned" the action, and she denied any involvement by Lamont or members of his campaign. She added that the Lamont campaign did not know who was behind the incident.
McKinney loses runoff
In Georgia, Rep. Cynthia McKinney -- a vehement critic of President Bush who scuffled with a Capitol Police officer in March but was not charged -- lost in a runoff against Hank Johnson, a former county commissioner in her suburban Atlanta district.
Johnson thumped McKinney by a margin of 59 percent to 41 percent but McKinney was unbowed. (Full story)
McKinney, an outspoken liberal, calls herself Bush's "worst nightmare."
She lost her bid for a sixth term in Congress in 2002, after she suggested that members of the Bush administration stood to profit from the war that followed the September 11, 2001, attacks on New York and Washington -- but won the seat back in 2004.