The Dodd endorsement is the more surprising of the two, with the Connecticut senator lagging behind better-known rivals Clinton, Illinois Sen. Barack Obama and John Edwards in the latest polls.
The firefighters' executive board will officially vote and endorse Dodd on Wednesday. Dodd and the firefighters plan a three-state tour, visiting Iowa on Thursday, New Hampshire on Friday and Nevada on Saturday.
"There's going to be lightning in the cornfields of Iowa," the Connecticut senator predicted, as he accepted the firefighters union's endorsement.
In 2003, the 281,000-member union backed Democratic Sen. John Kerry, a boost to his then-moribund campaign that helped him secure the 2004 party nomination, and its backing of Dodd is certain to improve his standing.
The firefighters "know how to win elections, and they'll put lots of boots on the ground. This is an important election, and they know it," Dodd said. "They've made it clear it would be about the future - who could win the election, fight for middle class and bring people together to get the job done."
IAFF President Harold A. Schaitberger said Dodd earned their support because he's the best candidate, and the 25-year Senate veteran has been a longtime ally of firefighters in Congress.
"He's done more than vote right and be supportive - he's really carried our water and been proactive on our behalf," Schaitberger said. "And the other reason is that I really think he has the experience. Our board gave a lot of thought to this. He has the experience and the strength to lead this country in what we know is going to be tough times."
Clinton got the endorsement of the 125,000-member United Transportation Union on Tuesday.
"The UTU has a long history of picking winners early. Hillary will be a president that America's working families can count on. Time and again, as a United States senator, she has stood with us," UTU President Paul Thompson said.
Clinton said she's honored to get the endorsement.
"America's workers have been invisible to this administration, and it's time they had an advocate in the White House," said the New York senator.
The UTU, which calls itself the largest railroad operating union in North America, represents 125,000 active and retired members in the railroad, bus and public transit industries.
The International Association of Fire Fighters represents more than 281,000 professional firefighters and emergency medical personnel in the United States and Canada. Firefighters emerged from the Sept. 11, 2001, attacks on New York City and the Pentagon as heroes, leading all eight Democratic candidates to work for their endorsement.
Dodd has the best chance to win the battleground votes outside the traditionally Republican and Democratic loyalists, Schaitberger said.
"The battleground has been and I believe will be the sliver in the middle, and that sliver in the middle demographically looks like, sounds like, and is ideologically and philosophically, like my membership. And Chris Dodd is the candidate who can speak to them," Schaitberger said.
The firefighters union gave $2.1 million in political donations in 2006 with 72 percent going to Democrats, according to the Center for Responsive Politics. The IAFF gave $1.78 million in the last presidential election cycle, with the same amount - 72 percent - going to Democrats.
Clinton's strong performance at the AFL-CIO candidate forum in Chicago and her support for rebuilding the nation's infrastructure were key to UTU's decision, said the group's incoming president, Mike Futhey Jr.
"Hillary Clinton's record has been friendly to working men and women of this country. She consistently has endorsed the necessity of a strong middle class," Futhey said.
The UTU contributed $1.3 million in the 2004 federal elections, with 84 percent of the money going to Democratic candidates. The union gave $1.2 million in the 2006 elections, with 89 percent directed to the Democrats in a year in which the party reclaimed the House and Senate from the Republicans.
Several unions are expected to jump into the Democratic nomination fray as Labor Day approaches. The AFL-CIO, the nation's largest labor federation, has decided not to immediately endorse any of the Democratic candidates, freeing the federation's 55 member unions to endorse whoever they want.
In the 2004 elections, organized labor gave $53.6 million to Democratic candidates and party committees. That amount increased to $66 million for the 2006 elections and is expected to increase again for 2008.