"I don't want a campaign that is divisive here, and there's a danger in that," Dodd said, although he denied he was nudging Sen. Hillary Rodham Clinton to end her candidacy.
Dodd said Obama was "ready to be president and I am ready to support him in this campaign."
The two men appeared together at a news conference. Dodd is the first of the Democratic campaign dropouts to endorse another candidate.
He said Obama "has been poked and prodded, analyzed and criticized, called too green, too trusting and for all of that has already won" more than half the states and millions of votes.
"It's now the hour to come together. ... This is the moment for Democrats and independents and others to come together, to get behind this candidacy," he said.
Dodd said he spoke with Clinton on Monday evening to tell her of his decision.
Dodd said he wasn't worried that the candidates would go too far in their pursuit of victory, but that their aides and supporters might.
"We've witnessed a little bit of that" in recent days, he said.
That was an apparent reference to a photograph that shows Obama wearing a white turban and a wraparound white robe that was presented to him by elders in Wajir, in northeastern Kenya.
The gossip and news Web site The Drudge Report posted the photograph Monday and said it was being circulated by "Clinton staffers" and quoted an e-mail from an unidentified campaign aide. Drudge did not include proof of the e-mail in the report.
The Clinton campaign has said it did not sanction circulation of the photo.
Obama told reporters, "I don't think that photograph was circulated to enhance my candidacy, I think that's fair to say.
"... Do I think that is reflective of Senator Clinton's approach to the campaign, probably not."
Associated Press Write Andy Miga in Washington contributed to this report.
© 2008 Associated Press