He told reporters earlier this week he hoped the joint appearance in New Hampshire signalled an active role for the New York senator in his White House race.
Clinton, who entered the race in January 2007 a heavy favourite, battled Obama to the end of voting on June 3 in a race that embittered some of her supporters -- particularly some of the women who formed her core constituency.
Whether Obama can win over all of those supporters remains unclear, but many public opinion polls show him solidifying his Democratic backing in recent weeks and moving out to a clear national lead on McCain.
Clinton, who returned to her Senate duties in Washington this week for the first time since dropping out of the Democratic race on June 7, offered strong praise for Obama.
"I've had a front row seat to his candidacy," she said. I've seen his strength and determination, his grace and his grit."
Clinton introduced Obama to her top fundraisers at a private meeting in Washington on Thursday night. Obama already had asked his big donors to help her pay off more than $10 million (5 million pounds) she owes to campaign debtors.
Obama, his wife Michelle, and finance committee chairwoman Penny Pritzker, along with other top Obama donors, each gave Clinton the maximum $2,300 check to help her pay off her debt. Obama will not ask his grass-roots list of 1.5 million donors to kick in on the Clinton debt.
Clinton and her husband, former President Bill Clinton, donated the maximum to Obama's campaign, Clinton aides said.
The two senators flew to Manchester, New Hampshire from Washington together on Obama's campaign plane, sitting in the second row together and chatting amiably for the hour-long flight.
They also shared a bus with a half-dozen aides for the 90-minute ride from Manchester to Unity. Obama spokeswoman Linda Douglass said the bus conversation was light-hearted banter on foods and computers, but the two did adjourn for a private discussion before speaking at the rally.
The tiny town of Unity in New Hampshire, near the Vermont border, was chosen for the rally not only for the obvious symbolism of its name but because the state will be a critical battleground in the race with McCain.
Democrat John Kerry narrowly captured New Hampshire in the 2004 race against Republican President George W. Bush, but Bush beat Democrat Al Gore there in a close race in 2000.
