Former Rep. Martin Frost (D-Texas) began informing party leaders Tuesday that he is dropping out of the race to chair the Democratic National Committee, sources said — a move that adds further momentum to the frontrunning bid of former Vermont Gov. Howard Dean for the post.

Frost was expected to tell Dean of his decision this afternoon, but it was not clear whether he would endorse the former governor.

Frost’s office did not immediately return a call seeking comment on his imminent departure. But Congressional sources suggested that Frost’s decision was prompted by an inability to secure the support of organized labor and other key party leaders.

Frost is the second candidate to drop from the DNC chair race just this week. On Monday, former Denver Mayor Wellington Webb left the contest, conceding that Dean has the votes to win.

With the departures of Frost and Webb, the field has slimmed to only Dean, New Democrat Network President Simon Rosenberg, political operative Donnie Fowler, former Ohio Democratic Party Chairman David Leland and former Rep. Tim Roemer (Ind.).

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 grassroots 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. 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 hampered by organized labor’s unwillingess 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, 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 that the race appears to be all but over.