Hoffa met with Obama on Wednesday in Texas, site of the next Democratic primary against Sen. Hillary Rodham Clinton. The Teamsters endorsement is expected to help Obama there and in Ohio on March 4, as well as in Pennsylvania on April 22.
The Teamsters have 80,000 members in Pennsylvania, 60,000 in Ohio and 17,000 in Texas, Hoffa said. Besides those members on the ground, the Teamsters plan to have their members and their families from around the country work for Obama, Hoffa said.
"We're going to say, 'Yes, yes, we can elect Barack Obama,"' Hoffa said. " ... He's got the best chance to win in the November elections."
Obama's wins Tuesday in Wisconsin and Hawaii helped seal the endorsement, Hoffa said. Clinton has now lost 10 contests in a row to Obama.
"We see the trends and we're impressed by the momentum of the Barack Obama campaign," said Hoffa, who noted that the union's executive board voted unanimously to endorse Obama. Internal polling of the union's members also showed they preferred Obama and that they thought he would do better against presumptive Republican presidential nominee John McCain, Hoffa said.
Hoffa said he planned to call Clinton and her husband, former President Clinton, to tell them why the union made its decision. "I owe it to them," said Hoffa, who said he had been lobbied by the former president to endorse his wife.
The Democratic presidential contenders have lobbied hard for the Teamster endorsement because of the power the union wields through its fundraising for Democratic candidates and get-out-the-vote programs. The Teamsters gave more than $2.2 million to Democrats in federal races in 2004. They have given more than $24 million to Democratic election causes since 1989, according to the Center for Responsive Politics.
Ohio and Pennsylvania have some of the largest number of union workers, with more than 15 percent of the work force unionized in Pennsylvania and more than 14 percent in Ohio.
The endorsement from the Teamsters is Obama's fourth from organized labor in a week. The 65,000-member International Brotherhood of Boilermakers endorsed Obama on Wednesday, the 1.9-million member Service Employees International Union backed the Illinois senator last Friday, and the smaller United Food and Commercial Workers endorsed him last Thursday.
Clinton has a larger number of unions in her corner with 12 endorsements from unions affiliated with the AFL-CIO - the nation's largest labor federation - and the United Farm Workers from the rival Change To Win labor federation.
But Obama also has two AFL-CIO unions in his corner - the Transport Workers Union and the United Association of Plumbers and Pipefitters. He also has the backing of the independent National Weather Service Employees Organization. And with the Teamsters endorsement, he will have four Change To Win unions in his corner: the Teamsters, SEIU, the United Food and Commercial Workers and UNITE HERE, which gave Obama his first major union endorsement.
Four of the seven Change to Win unions must endorse a candidate for the federation to consider its own endorsement. The federation's leaders planned to discuss a possible endorsement Thursday morning, officials said.