Sen. John Kerry (Mass.) became the third prominent Democrat to eschew the 2008 presidential race when he announced from the Senate floor Wednesday that he would seek re-election instead.

The 2004 White House nominee joins Sen. Russ Feingold (Wis.) and former Virginia Gov. Mark Warner on the sidelines as the 2008 presidential contest takes shape at warp speed.

The decorated Vietnam War veteran said he would work on his No. 1 priority, finding a successful conclusion to the Iraq War, from the Senate rather than the presidential campaign trail.

He also launched a new Web site,

www.setadeadline.com, to pressure President Bush into establishing a deadline for withdrawing U.S. troops from Iraq.

By announcing his intention to seek a fifth term, Kerry dashed the hopes of several of his Democratic Bay State colleagues in the House who were hoping to succeed him in the Senate.

Sen. Edward Kennedy (D-Mass.), who was first elected to the Senate in 1962, is not up for re-election until 2012. Kerry, who was first elected in 1984, is a heavy favorite for re-election next year, when he will be 64.

Nonetheless, those Members who began jockeying for pole position in 2004 when they thought Kerry would win the presidency and vacate his seat, were gracious about the news.

“His decision now to focus his energy on continuing to serve the Commonwealth of Massachusetts is a disappointment to his legion of supporters [who wanted him to run for president] but preserves one of the most influential voices in the U.S. Senate working on behalf of our great state, the nation, and the world,” said Rep. Ed Markey, the dean of the Bay State’s delegation in the House, who amassed more than $2 million in his House campaign account in the event Kerry left the Senate.

Rep. Marty Meehan, who ended 2006 with nearly $5 million in the bank, conceded that he “probably was positioned well” to run for Senate. Nonetheless, he said he is not “disappointed” about Kerry’s decision.

“I don’t think these things are personal; I think John Kerry does a terrific job for the people of Massachusetts,” Meehan said.

As for Meehan’s own political future — he has been rumored to be eyeing several jobs in academia and the private sector — the Congressman said he enjoys serving in the House.

“We’re lucky to have both Sen. Kerry and Sen. Kennedy,” he said. “We just had the election in 2006 that put the Massachusetts Democrats in the House in a very strong position; we’re probably among the most effective delegations in the country.”

A spokesman for Meehan denied recent local press accounts that suggested the Congressman has plans to leave Capitol Hill.

Meehan, Markey, Rep. Stephen Lynch (D-Mass.) and others who were waiting in the wings probably are relieved to have an answer one way or the other, said one Massachusetts Democratic staffer, who did not want to be named.

“People are just glad that the decision has been made,” the source said. “It’s been a torturous process, the waiting, waiting, waiting.”

The midterm elections also helped satisfy some Members anxious to move up — for example, Rep. Barney Frank (D-Mass.) now leads the House Financial Services Committee.

“The new majority in the House takes some of that sting away,” the source said.

Those close to Kerry said he easily could have run for president again.

“It was a tough decision,” said one insider, who did not wan to be named. “He has $12.5 million in the bank; he could have not raised another dime this quarter and had more than the rest of the field.”

After his White House loss, Kerry became a prolific fundraiser for his party and dogged campaigner for Democratic candidates up and down the ticket.

He raised $14 million for 260 candidates and campaigned in 35 states in the previous cycle.

“He’s got a really active, national political organization,” the source said.

Meehan said Kerry has “boundless” energy and that he expects the state’s junior Senator to continue helping Democrats across the country.

Considering his 3-million-person strong e-mail list, amassed during the presidential campaign and carefully maintained by his political action committee since, Kerry certainly has reach.

“I do think he’s positioned really well,” Meehan said. “Massachusetts will continue to have a national Senator.”

Kerry made clear that he wants to stay on the national stage.

“We came close, certainly close enough to be tempted to try again,” Kerry said during his Senate floor speech. “There are powerful reasons to try to continue that fight now, but I’ve concluded that this is not the time for me.”

Kerry said he feels an obligation to fight against the war from his Senate perch.

“As someone who made the mistake [of voting for the war] I feel the weight of personal responsibility to act,” he said.

Ironically, it was a self-described “botched joke” in the fall about soldiers serving in Iraq that may have dimmed Democratic activists’ enthusiasm for another Kerry presidential bid.

In a video clip posted Wednesday on JohnKerry.com, Kerry told his supporters that he will “do all I can to end this war and strengthen our security.”

In a sign of how serious Kerry is about his new mission, he has tapped Jerome Armstrong, a darling of liberal activists and the “net roots” — a phrase Armstrong coined — to lead setadeadline.com, according to a source close to Kerry.

Armstrong founded the liberal blog, Mydd.com. Since then he has become a Democratic strategist and consulted the campaigns of Sen. Sherrod Brown (D-Ohio) and New Jersey Gov. Jon Corzine (D). Armstrong also advised Warner on Internet strategy for his leadership political action committee, Forward Together.

Other Democratic presidential aspirants sang Kerry’s praise once their potential rival was out of the hunt.

“From his earliest days in Vietnam to the presidential election in 2004, John Kerry has fought for his country and his ideals,” Sen. Barack Obama (D-Ill.) said in a statement. “I am proud to call him a friend and a colleague … and know that he will continue to serve his country with honor and distinction in the years to come.”

Kennedy, who immediately followed Kerry on the Senate floor, sounded sad about Kerry’s decision.

“His eloquence, his passion, his insights … characterizes the career of John Kerry,” said Kennedy, whose coveted endorsement in the presidential race is now up for grabs. “Now John has decided to continue to devote his passions, his energies, toward bringing our troops home from Iraq safely and how fortunate that they are to know that he will devote his energies to the cause.”

Senate Majority Leader Harry Reid (D-Nev.) said Kerry would have made an excellent president.

“I love you, John Kerry, and I’m sorry that things didn’t work out for our country,” Reid said minutes after an emotional Kerry surrendered the floor.