Former Texas Rep. Martin Frost’s decision to remove himself from the race for Democratic National Committee chairman effectively ends a political career that spanned nearly three decades and all but ensures that former Vermont Gov. Howard Dean will ascend to the post later this month.

Frost began informing supporters and party leaders Tuesday afternoon that he would not continue in the contest and has said repeatedly during the DNC race he would never again seek elective office.

Although Frost offered no explanation for his departure in a statement released by his campaign, informed sources suggested that the decision by the AFL-CIO not to endorse him — or any other candidate — ended Frost’s chances to challenge Dean for the chairmanship.

Rep. Mike Honda (Calif.) did win the backing of the AFL-CIO for his DNC vice chairmanship race, in which he is competing against Rep. Gregory Meeks (N.Y.) among others.

Frost had also struggled to secure significant backing from his former colleagues, especially House Minority Leader Nancy Pelosi (Calif.).

Frost’s exit from the race came just one day after former Denver Mayor Wellington Webb dropped out; former Ohio Democratic Party Chairman David Leland called it quits just minutes after Frost, throwing his support to Dean.

Sources indicated Tuesday that New Democrat Network President Simon Rosenberg had also suspended his campaign, but a spokesman denied he was out.

“We are assessing the situation and at this point — we are moving forward,” said Gil Meneses.

Regardless, Frost seemed to acknowledge the inevitability of a Dean victory in his statement, noting that “the challenge ahead for Governor Dean will be to unite the party, rebuild the DNC and win elections in every region of the country.”

Aside from Dean, only former Indiana Rep. Tim Roemer and political operative Donnie Fowler remain in the DNC contest.

Dean refused to bask in his seeming victory Tuesday, saying only: “While I am encouraged by the news of the day, this race is still not over.”

In all, 447 voting members will select a new chair during the DNC’s winter meeting, which is set for Feb. 10-12 in Washington, D.C.

Since his entry into the race, Dean has been the frontrunner thanks in large part to the loyal following of a huge grass-roots movement developed during his run for the Democratic presidential nomination in 2003 and 2004.

As happened in the presidential race, a number of establishment Democrats have been working behind the scenes to coalesce support behind a single anti-Dean candidate, which Frost had hoped to be. The anti-Dean wing fears that the former governor is too closely identified with the ideological left to be an effective national spokesman and strategist.

But sources say the recruitment of an anti-Dean “unity” candidate has been hamstrung by organized labor’s unwillingness to actively oppose the apparent juggernaut.

Without Frost in the race, Fowler, who ran the Michigan operation for the presidential campaign of Massachusetts Sen. John Kerry in 2004 and is the son of former DNC Chairman Don Fowler, may become the default choice for those who want to derail Dean.

Even many Democrats on Capitol Hill who are not enamored with the prospect of the fiery former governor as the face of their party for the next several years acknowledged on Tuesday, however, that the race appears to be all but over.

What is almost certainly over is the political career of Frost, which began when he was elected to a northern Texas seat in 1978, at age 36.

“Throughout the campaign he has said he wasn’t running for elected office again,” Frost spokesman Tom Eisenhauer said Tuesday.

Frost ended his career with a string of defeats after a rapid rise into the upper echelon of House leadership. He chaired the Democratic Congressional Campaign Committee in the 1996 and 1998 cycles to much acclaim and went on to serve as Democratic Caucus chairman.

Frost made no secret of his interest in serving as the leader of his party whenever Rep. Richard Gephardt (Mo.) stepped down from the post, which Gephardt eventually did following the 2002 election.

What had been a years-long behind-the-scenes struggle went public for one day in late 2002 as Frost and Pelosi scrambled for supremacy. Just more than 24 hours after he entered the race Frost bowed out, acknowledging that Pelosi had the votes.

That pattern — a quick exit following an analytical assessment of his chances — is vintage Frost, according to those close to him, and was repeated in his decision to step out of the DNC fray.

After successfully engineering a Democratic majority in the Texas Congressional delegation following the 1980 and 1990 censuses, Frost fell victim to a GOP-led re-redistricting of the state’s lines in 2003.

Under the plan engineered by House Majority Leader Tom DeLay (Texas), Frost saw his old 24th district seat split between five Congressional districts.

He eventually chose to challenge Rep. Pete Sessions (R) in the 32nd, citing its large Hispanic population.

Though Frost raised more than $4 million and polling showed the race close, he wound up losing by 10 points.

Some had speculated that Frost would run against Sessions in 2006, but his statements during the DNC campaign seemed to put an end to that speculation.