Iowa Sen. Tom Harkin (search) endorsed Howard Dean (search) for the Democratic presidential nomination on Friday, giving the front-runner's campaign a critical boost ahead of Iowa's first-in-the-nation caucuses.

"This year, we have an exceptionally strong field of candidates. Each one is a talented, well-qualified Democrat who would be a vast improvement over the current occupant of the White House. I like and respect each one of them," Harkin said to cheers at Dean's headquarters in Des Moines.

"But for me, one candidate rose to the top as our best shot to beat George W. Bush and to give America the opportunity to take our country back. That person is Governor Howard Dean," he said.

The backing of Harkin, who ran for president in 1992 but lost in the primaries to Bill Clinton, comes as Dean confronts a barrage of criticism. Iowa's caucuses are scheduled for Jan. 19.

The former Vermont governor, campaigning from New Hampshire, welcomed the backing of Harkin, saying he always admired him "because he's a real fighter and a real scrapper."

Political pundits say support from the four-term senator and most sought-after Iowa lawmaker gives Dean the backing of the state's most durable Democratic politician, and a man whose organization can prove a vital asset on caucus night. Harkin has organized several forums in Iowa for voters to meet the candidates.

"There's no question it's a significant endorsement," said Republican National Committee spokesman Jim Dyke. "But politics is an up and down game ... from day to day it's different, it changes, it evolves."

Dean and his rivals for the nomination had all appealed to Harkin for backing, and the senator had said publicly in recent days that he was weighing whether to choose sides, or perhaps remain neutral.

On Friday, Iowa Attorney General Tom Miller (search), the top statewide vote-getter in Iowa in 2002, endorsed Massachusetts Sen. John Kerry (search).

In an interview with the Associated Press, Harkin, who shares much of Dean's political leanings, called Dean "the Harry Truman of our generation. Howard Dean is really the kind of plain-spoken Democrat we need."

Harkin also raised Dean for bringing a breath of fresh air to the race.

The endorsement comes as Dean tries to tamp down an uproar over comments he made four years ago to a Canadian news program in which he said the Iowa caucuses are dominated by extremists and are a waste of time.

"If you look at the caucuses system, they are dominated by the special interests, in both sides, in both parties. The special interests don't represent the centrist tendencies of the American people. They represent the extremes," he said on "The Editors," a Montreal-based program that Dean frequently appeared on while governor of Vermont.

On Friday, he made an about-face on those sentiments.

"Four years ago, I didn't really understand the Iowa caucuses," Dean said Friday. "I wouldn't be where I am without the Iowa caucuses."

The latest Iowa poll showed Dean leading 29-25 over his closest rival in the state, Missouri Rep. Dick Gephardt (search). According to the four-point margin of error on the sample, the two are in a statistical tie.

Gephardt has flatly predicted he will win Iowa, and his aides concede he must. Kerry, long ago a national front-runner who has frittered away his lead, is looking to Iowa to reinvigorate his candidacy.

With its leadoff position on the election calendar, Iowa often takes on outsized importance in the race for a presidential nomination. A victory can lend the winner momentum going into the New Hampshire primary eight days later. A defeat, or a weak showing, can be hard to overcome.

Harkin said this year's Democratic race has been one of Iowa's best.

"This year, the enthusiasm and energy surrounding the caucuses is really incredible. Iowans are fired up to take back the White House," he said.

Fox News' Steve Brown and The Associated Press contributed to this report.