Democratic front-runner Barack Obama has scored a coup in his White House nominating battle against Hillary Clinton by winning the high-profile endorsement of blue-collar champion John Edwards.

Edwards, a two-time presidential hopeful and the party's 2004 vice presidential nominee, formally backed the Illinois senator at an exuberant Michigan rally late Wednesday, reinforcing signs of the Democratic establishment closing ranks behind Obama.

The announcement punctured Clinton's short-lived boost after her landslide win in Tuesday's West Virginia primary, although the former first lady said she didn't believe in quitting, and would fight on.

Edwards hailed Clinton as a woman "made of steel" who had strengthened the Democratic Party and the eventual presidential nominee by fighting so doggedly for issues dear to her heart.

But the former North Carolina senator said: "The Democratic voters of America have made their choice and so have I."

Clinton campaign chairman Terry McAuliffe brushed off Edwards' announcement, which capped a day of endorsements for Obama including that of the nation's top abortion rights organization.

"We respect John Edwards, but as the voters of West Virginia showed last night, this thing is far from over," he said.

The former first lady routed Obama by 67 to 26 percent in West Virginia and was the runaway leader with white and lower-income voters, in a worrying sign for Obama as he contemplates swing voters Democrats need to win in November.

Asked earlier on CNN if she would fight to the end of the primary season on June 3, Clinton said: "I'm not going anywhere, except to Kentucky and Oregon, and Montana, South Dakota, and Puerto Rico."

"I don't believe in quitting. You may not win in life, but you do the best you can. You go the distance," she said, after scooping 20 of West Virginia's 28 delegates on Tuesday.

However, far more significant than West Virginia is the bloc of nearly 800 Democratic "superdelegates" who could have the casting vote to decide the party's White House standard-bearer against Republican John McCain.

Obama won the support of at least five more superdelegates Wednesday. Clinton secured the backing of one, a party leader in Tennessee.

Obama was also endorsed by three former chairmen of the Securities and Exchange Commission, who joined ex-Federal Reserve chief Paul Volcker in extolling the Democrat's capacity to take on "monumental economic challenges."

And Clinton, bidding to be the first female president, suffered a body blow with the endorsement of her rival by the million-strong NARAL Pro-Choice America.

The group's president, Nancy Keenan, said its vaunted grassroots strength was going to Obama, "the pro-choice candidate whom we believe will secure the Democratic nomination and advance to the general election."

According to the latest tally by the independent website RealClearPolitics, Obama now has 1,886 delegates in total to Clinton's 1,719, putting him considerably closer to the winning line of 2,025.

A new national poll by Quinnipiac University said 60 percent of Democrats want the Illinois senator to pick Clinton as his vice presidential running mate. Both candidates have declared such talk to be premature.

The poll also showed Obama leading McCain in a November match-up, by 47 percent to 40. But 63 percent of Democrats wanted Clinton to stay in the primary race .

Edwards quit this year's White House nominating race in late January but remains an outspoken voice on the need to confront America's growing rich-poor divide.

He could now release his 19 pledged delegates to Obama, and help him reach out to the white, working-class voters who have flocked to Clinton's flag and who will play a key role in November's general election.

Earlier in Michigan, Obama rolled out a 210-billion-dollar plan to kick-start US manufacturing including Detroit's beleaguered auto industry.

He said McCain had been right to state in January that lost manufacturing jobs would not be regained.

"Where he's wrong is in not offering new solutions or economic policies that are different from what (President) George Bush has given us for eight long years. That's wrong. That's giving up," he said.

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