WASHINGTON — John Edwards’s endorsement of Barack Obama gives the Illinois senator a powerful ally as he seeks to unify the Democratic Party around his candidacy and push Senator Clinton out of the presidential race.

The former North Carolina senator and two-time White House hopeful threw his support to Mr. Obama at a rally yesterday in Michigan, saying that among the candidates, he is “the one man who knows in his heart that it is time to create one America, not two.”

For Mr. Obama, the endorsement was timed to head off any momentum Mrs. Clinton hoped to gain from her blowout victory in the West Virginia primary Tuesday.

Mr. Edwards had sat on the sidelines for three months after leaving the race in February, resisting aggressive courtships from Mr. Obama and Mrs. Clinton. He had indicated that Mr. Obama was the likely nominee in recent days, suggesting an endorsement of Mrs. Clinton was not in the cards.

The two Democrats appeared together at a large rally in Grand Rapids, Mich., where Mr. Obama is reaching out to voters in a key general election state where party rules barred him from campaigning during the primary season.

Mr. Edwards opened his speech with an extended tribute to Mrs. Clinton, praising her “strength and character” and saying she “is a leader in this country not because of her husband but because of what she has done.”

Like other party leaders who have endorsed Mr. Obama in recent days, however, he signaled that the primary campaign was all but over.

“The reason I am here tonight is that Democratic voters in America have made their choice, and so have I,” Mr. Edwards said.

Mr. Obama introduced Mr. Edwards as “one of the greatest leaders we have in the Democratic Party,” and he committed in his speech to supporting Mr. Edwards’s campaign to reduce poverty by half in the next decade.

Notably absent yesterday was Elizabeth Edwards, who has not endorsed a candidate and who has criticized Mr. Obama’s health care plan, which does not require all Americans to purchase insurance, as do the proposals offered by her husband and Mrs. Clinton.

Mr. Edwards’s backing carries both symbolic and mathematical significance. He earned 19 pledged delegates during the primary campaign, whom he now will likely urge to back Mr. Obama. Those delegates are not bound to follow Mr. Edwards’s lead, but those who do will push Mr. Obama even closer to clinching the nomination.

Mr. Edwards may also help Mr. Obama connect with white working-class voters, whose strong support of Mrs. Clinton has raised questions about whether Mr. Obama can carry key states like Ohio and Pennsylvania in the general election. Mr. Edwards ran with a populist economic message during his campaign, and his lingering appeal was evident in rural West Virginia, where he received 7% of the vote Tuesday even though he had long since exited the race.

Although Mr. Obama has refused to declare victory or even suggest Mrs. Clinton end her campaign, he and his aides appeared to carefully choreograph the Edwards endorsement to upstage her efforts to capitalize on the West Virginia rout. The former first lady conducted interviews for the network evening new shows, but Mr. Obama timed his endorsement rally to begin during the same broadcasts.

An Obama aide said last night that while Messrs. Obama and Edwards have spoken regularly in recent months, the 2004 vice presidential nominee did not inform him of his endorsement decision until late Tuesday night. Mr. Obama had called him after the West Virginia results, and Mr. Edwards agreed to appear at his side in Michigan yesterday, the aide said.

As late as Monday, Mr. Edwards told CNN he had no plans to announce an endorsement.

The chairman of Mrs. Clinton’s campaign, Terry McAuliffe, said in a statement: “We respect John Edwards, but as the voters of West Virginia showed last night, this thing is far from over.”

Mrs. Clinton reiterated in several television interviews yesterday that she intended to stay in the race through the final contests on June 3, arguing that she could persuade the remaining undecided superdelegates that she was the strongest candidate to face Senator McCain this fall.

She suffered another stinging setback yesterday, however, when a major abortion rights group, NARAL Pro-Choice America, endorsed Mr. Obama. The organization’s president, Nancy Keenan, praised both contenders but said in a statement that Mr. Obama was “the pro-choice candidate whom we believe will secure the Democratic nomination and advance to the general election.”

The move appeared to catch the Clinton campaign off-guard and sparked outrage among some of its top supporters. Asked for his reaction during a conference call with reporters, Mrs. Clinton’s chief spokesman, Howard Wolfson, said: “Surprised. Senator Clinton’s leadership and advocacy on choice issues is second to none.”

Ellen Malcolm, a prominent Clinton backer and the president of Emily’s List, a group that supports women candidates, called the decision by NARAL “tremendously disrespectful.”

More than a dozen members of Congress supporting Mrs. Clinton gathered on Capitol Hill yesterday afternoon to denounce the move and tout her record on abortion rights.

Mr. Obama yesterday also picked up the endorsement of five more superdelegates, while Mrs. Clinton earned one.