Democratic Sen. Tom Harkin (search), once thought unlikely to pick a favorite in the Democratic presidential contest, is now considering an endorsement, aides and associates say.

Harkin, whose backing is actively being sought, has spoken directly with most of the top candidates in the nine-person Democratic field, as well as with labor leaders who are an important part of his base, aides and associates say. A decision is likely soon.

"They've all called here in the past few days," Harkin spokeswoman Allison Dobson said Monday.

A Harkin endorsement could make the difference in the caucuses, which are two weeks away and remain competitive. The campaign of front-runner Howard Dean (search) has been pursuing a Harkin endorsement. Rival Rep. Dick Gephardt (search) of Missouri has the backing of more unions than Dean, and labor is a key part of Harkin's political base.

"It's a 50-50 proposition right now," Dobson said of an endorsement by Harkin, perhaps the state's most popular Democrat.

Harkin breezed to a fourth term in the Senate in 2002 and held a series of town-hall forums with the candidates last year. His political organization in the state is formidable, and he hasn't been shy about using it to aid political allies.

The move comes as Iowa's Democratic Gov. Tom Vilsack (search) told aides he will remain neutral, as he did in the 2000 Democratic race. Harkin, who briefly sought the nomination in 1992, is the more coveted prize this year for the Democratic hopefuls.

Harkin endorsed Gore in 2000 and offered his political organization to that camp. He was credited with helping Gore turn back a challenge from then-Sen. Bill Bradley in Iowa's caucuses.

Several of Harkin's Senate colleagues also are battling in Iowa, including Sen. John Kerry (search) of Massachusetts and Sen. John Edwards (search) of North Carolina.

If Harkin comes to the conclusion that Dean is building a lead and headed for the nomination, a pre-caucus endorsement would be crucial to maintaining his clout with the campaigns as the race proceeds.

Vilsack spokeswoman Amanda Crumley said the governor "feels it's important to keep the door open in Iowa to all Democrats."

Crumley said Vilsack told staffers Monday there would be no endorsement, in part to assure a fair contest and boost the state's chances of retaining its first-in-the nation status. The state is competitive in the general election, and Vilsack wants to stay out of the fray to make it easier to heal any wounds in the fall.