NEW YORK - Former US presidential candidate John Edwards endorsed Senator Barack Obama on Wednesday in an effort to transfer his appeal among white, working-class voters to the Democratic front-runner who has struggled to win them over. Obama's campaign hopes the endorsement, late in the process, will help buttress the aura of inevitability of his quest for his party's nomination. It also served as a strong counterpoint to the contention by Senator Hillary Clinton that Obama's weakness among blue-collar workers could cripple his effort in November's presidential election. While the value of any endorsement is always subject to scrutiny, both candidates had aggressively sought support from Edwards, the Democratic vice presidential nominee in 2004 and a favourite among union voters. The two one-time rivals stood together on a stage in Grand Rapids, Michigan, where Edwards praised Mrs. Clinton before explaining his decision to back Obama. "The reason that I am here tonight is because the Democratic voters in America have made their choice, and so have I," Edwards said. "There is one man who knows and understands that this is a time for bold leadership. There is one man that knows how to create the change, the lasting change, that you have to build from the ground up." The backing from Edwards, the son of a millworker, could give Obama, an African-America, a significant boost not just with a key demographic but also with the party leaders and elected officials who ultimately are expected to determine the nomination through their status as superdelegates. The announcement overshadowed interviews Mrs. Clinton did with several TV networks that focused on her 41-point victory in Tuesday's West Virginia primary, a win that reignited questions about Obama's ability to attract white, working-class voters. Before leaving the race, Edwards had won 19 national convention delegates. He has the power to release them and presumably will encourage them to vote for Obama, further widening his delegate advantage. Edwards called for party unity once a nominee is chosen, as he spoke to an audience that Obama's campaign said exceeded 12,000. "When this nomination battle is over, and it will be over soon, brothers and sisters we must come together as Democrats, and in the fall stand up for what matters for the future of America," Edwards, the former North Carolina senator, said. Mrs. Clinton's campaign greeted the endorsement by pointing to Tuesday's vote. "We have a lot of respect for Sen. Edwards, but as West Virginia voters showed last night, this race is far from over," Mrs. Clinton campaign chairman Terry McAuliffe said in a statement. Obama called Edwards "one of the greatest leaders we have in the Democratic Party," as he pledged to fight poverty and work for affordable health care for all-both signature issues for his newest supporter. The endorsement ended months of speculation over what Edwards would do. When he left the race in January, Obama's aides believed they would be lucky to simply keep him from endorsing Mrs. Clinton. Obama essentially ended Edwards' presidential hopes by beating him in Iowa, where Edwards had bet the farm on a win following a second-place finish there in 2004. Edwards' backing of Obama was a blow for a Mrs. Clinton campaign that readily adopted his populist themes in recent weeks. During his run for the presidency, Edwards had criticized Obama for not being tough enough to challenge the special interests in Washington and for proposing a health-care expansion plan that he said was not truly universal. At the same time, Edwards had been critical of Mrs. Clinton's acceptance of campaign donations from federal lobbyists-donations that Obama rejected in his presidential campaign.