DES MOINES — Democratic presidential candidates have begun concentrating their fire on leader Howard Dean, often forcing him into a defensive posture.

But the former Vermont governor received one of the biggest pushes of his campaign Friday: the endorsement of Sen. Tom Harkin, D-Iowa.

“One candidate rose to the top as our best shot to take back the White House,” Harkin said Friday, announcing his decision at Dean’s campaign headquarters in Des Moines. “As we Iowans say, Howard Dean has his head screwed on right.”

Harkin’s announcement was one of several last week. Iowa Gov. Tom Vilsack said he would not endorse a candidate before the caucuses, House Minority Leader Rep. Nancy Pelosi, D-Calif., favored Rep. Dick Gephardt, D-Mo. and Iowa Attorney General Tom Miller gave his support to Sen. John Kerry, D-Mass..

But the support of the four-term Iowa senator, entering his 30th year in Congress, is likely to create the waves in the days leading up to the Jan. 19 Iowa caucuses, the first step in choosing a candidate to run against President Bush.

Christina Butts, caucus organizer in Des Moines and a Dean supporter, attended the Harkin rally and said she’d spoken to undecided Democrats in her precinct who might now be persuaded to join Dean’s cause.

“There are people who are on the fence; this might push them over the edge,” she said.

Harkin’s decision struck a strong blow to Gephardt’s campaign. Before a campaign stop Friday in Tama, Gephardt, who won the Iowa caucuses in 1988, told the Associated Press Harkin’s decision was a “disappointment” before adding that he still expects to win the Iowa contest.

Harkin’s endorsement could deflect criticism of Dean’s comments on a television show in 2000. In the show, Dean diminished the caucus system as one dominated by special interests.

Other Democratic candidates have torn apart various aspects of Dean’s political record and the sealing of his records as governor of Vermont, trying to erode his appeal before critical nomination contests in Iowa and New Hampshire. Dean also faces the continued charge of retired Army general Wesley Clark, a late entry into the presidential race.

Dean, Kerry and Gephardt are the leaders in Iowa as campaigning enters its last week.

Harkin ended his remarks in Des Moines by encouraging the Democratic field to follow the lead of Sen. John Edwards, D-N.C., by staying away from negative campaigning.

“Howard Dean will lead the way and I’m proud to be with him,” Harkin said before hoisting a orange “I Believe in Dean” T-shirt. “He is the Harry Truman of our time.”

— The Associated Press contributed to this story.