Senator Joe Lieberman has filed papers to stand as an independent in mid-term elections, a day after losing the Democratic Party's nomination.
Voters made Lieberman pay for his support for the Iraq war
"My mind is made up... I'm committed to this campaign," the veteran senator told NBC's Today television show.
Mr Lieberman says he easily garnered the 7,500 signatures needed to endorse his independent candidacy.
But his defeat in Connecticut to newcomer Ned Lamont was widely seen as an expression of anti-war feeling.
Mr Lieberman, a senator for 18 years, had been harshly criticised in his home state for his support for the Iraq war, and his perceived closeness to Republican President George W Bush.
In Tuesday's primary, Democratic voters were asked to decide which candidate would stand for the party in November's elections for the Senate and House of Representatives.
With most of the votes counted, results showed Mr Lamont - founder of a cable television company and a political novice - to have won with 52% compared to Mr Lieberman's 48%.
His defeat sent a message to politicians of all colours that Iraq is becoming a political liability, reports the BBC's Nick Miles in Washington.
Mr Lieberman had suggested he might run as an independent if he lost his ticket to run as the Democratic nominee in November, and he wasted no time, filing a petition to run on Wednesday morning.
1942: Born in Stamford, Connecticut
1970: Elected to Connecticut State Senate
1983: Elected as Connecticut attorney general
1989: Wins bid for US Senate
2000: First Jewish candidate for vice president
Unsuccessful bid for 2004 presidential nomination
His petition received 18,000 signatures, Mr Lieberman's campaign said - more than double the minimum of 7,500. Mr Lieberman has formed a new party for his candidacy, called Connecticut for Lieberman.
"I'll always take the calls of friends, but my mind is made up. I'm going forward. I'm going forward because I'm fed up with all the partisanship in Washington that stops us getting anything done," Mr Lieberman told the Today show.
Asked if there was anyone who could dissuade him, he said: "Respectfully no. I'm committed to this campaign."
Backing for Lamont
Meanwhile Senate Democratic leader Harry Reid of Nevada and Sen Chuck Schumer of New York - the chairman of the party's Senate campaign committee - have pledged their full support for Mr Lamont.
Leading Democrats have voiced support for Lamont
In a statement on the campaign committee website, they said: "The Democratic voters of Connecticut have spoken... Congratulations to Ned on his victory and on a race well run.
"Joe Lieberman has been an effective Democratic senator... But the perception was that he was too close to George Bush and this election was, in many respects, a referendum on the president more than anything else."
If the signatures on Mr Lieberman's petition for candidacy are approved, it sets up a race with Mr Lamont, the Republican candidate Alan Schlesinger and other candidates.