The Illinois senator and potential 2008 presidential candidate sent an e-mail message Thursday praising Lamont.
"Ned Lamont has waged an impressive grass-roots campaign to give the people of Connecticut a choice in the November Senate election," Obama wrote. "Please join me in supporting Ned Lamont with your hard work on-the-ground in these closing weeks of the campaign."
The Lamont campaign said Obama's e-mail went to about 5,000 Connecticut residents.
Lamont aides said they welcomed the support of Obama, who has enjoyed a surge in popularity in recent weeks as speculation about his national ambitions mounts. Obama has also given $5,000 to Lamont's campaign through a political committee.
"Ned Lamont and I share a commitment to bringing our troops home safely from Iraq, to achieving energy independence, to helping all our citizens realize the American dream, and to empowering the American people to reclaim their government," Obama wrote.
In the spring, Obama traveled to Connecticut to speak for Lieberman.
The three-term senator holds a double-digit lead over Lamont in recent polls. Lieberman, the Democratic nominee for vice president in 2000, lost the Democratic primary in August to Lamont. Lieberman is running as an independent. A third candidate, Republican Alan Schlesinger, trails far behind in the polls.
Lieberman, meanwhile, won praise from another prominent Democrat, Louisiana Sen. Mary Landrieu.
"One of the first calls I made after the levees broke was to Joe Lieberman," Landrieu said in a statement. "In a time of national crisis, Joe had the calm, intelligence and compassion to help get me — and our country — through those difficult months."
Lieberman is the ranking Democrat on the Senate's Homeland Security panel, which probed the government's response to Hurricane Katrina and made recommendations on how officials could do a better job.
Landrieu and Lieberman met voters and ate lunch on plastic plates at Hartford's Rajun Cajun restaurant. Landrieu enjoyed the jambalaya. Lieberman had macaroni and cheese.
Schlesinger, meanwhile, is airing three new TV ads he said will cost him $50,000 as the race closes.
"Have you noticed my two opponents seem to care more about the future of the Democratic Party than your future?" he says, speaking directly to the camera in one of the commercials. "I'm in this race because I believe that I can help you and your family. Not to advance some political agenda."