WRAPUP 5-Clinton scores big West Virginia win

(Adds results)

By John Whitesides, Political Correspondent

WASHINGTON, May 13 (Reuters) - Hillary Clinton scored a big victory over front-runner Barack Obama in West Virginia on Tuesday, slowing Obama's march to the Democratic presidential nomination but making barely a dent in his sizable lead.

Clinton's easy win in a state dominated by the white working-class voters who have been her biggest supporters gave her fresh ammunition to argue she is the Democrat with the best chance to beat Republican John McCain in November's presidential election.

But Obama, an Illinois senator, retains a nearly insurmountable advantage in delegates who will select the nominee at the party convention in August, and West Virginia had only 28 delegates at stake.

Clinton, a New York senator and former first lady, hopes her big victory in West Virginia will help fan doubts about Obama's ability to win important swing states in November. She has vowed to keep fighting despite her dwindling prospects and a mounting campaign debt.

"I wouldn't be doing this if I didn't believe that I could be the best president for West Virginia and America and that I was the stronger candidate to take on John McCain in the fall," she said at a rally in Logan, West Virginia on Monday.

But a newly minted Obama supporter, former Colorado Gov. Roy Romer, said it is now impossible for Clinton to overcome Obama's lead.

"The math is controlling. This race, I believe, is over," Romer said on a conference call sponsored by the Obama campaign.