By John Whitesides, Political Correspondent

WASHINGTON (Reuters) - Barack Obama took a commanding lead in the Democratic presidential race on Wednesday, but Hillary Clinton vowed to fight on after loaning her campaign $6.4 million.

Obama's big win in North Carolina and Clinton's slim victory in Indiana widened his advantage in their battle for the right to face Republican John McCain in the November presidential election with just six contests remaining.

The results left the cash-strapped Clinton campaign with little chance to halt Obama's march to the nomination. But the New York senator brushed off calls to drop out of the race.

"I'm staying in this race until there is a nominee," Clinton told reporters after a campaign rally in Shepherdstown, West Virginia, which holds a primary on Tuesday.

At a Washington fundraising event to honor women, she said she had been counted out before. "I am staying in this race," she said. "Too many people have fought too hard to see a woman continue in this race."

Clinton dipped into her personal fortune again to try to keep pace with Obama, putting $5 million into her campaign in April and $1.4 million over the past week, aides said.

"It's a sign of my commitment to this campaign," Clinton said of the loans.

She vowed to fight on to contests in West Virginia, and in Oregon and Kentucky on May 20, but Obama aides said he was closing in on the nomination.

"We believe we are going to be the nominee of this party," Obama campaign manager David Plouffe told reporters. He said the campaign would begin to look ahead when possible to a general election campaign against McCain.

Obama's 14-point victory in North Carolina was a dramatic rebound from a difficult campaign stretch that began last month with a big loss in Pennsylvania and was prolonged by the controversy over racially charged comments by his former pastor, the Rev. Jeremiah Wright.

With just 217 delegates at stake in the final six contests, Clinton has no realistic chance to overtake Obama's lead in pledged delegates who will help pick the nominee at the August convention. It is also nearly impossible to catch him in popular votes won in the state-by-state battle for the nomination that began in January.

California Sen. Dianne Feinstein, a Clinton supporter, told The Hill newspaper she wanted to find out about Clinton's remaining strategy. "I think the race is reaching the point now where there are negative dividends from it, in terms of strife within the party," she said.

An MSNBC count showed Tuesday's results expanded the Illinois senator's delegate edge by 12. He has 1,844 delegates to Clinton's 1,695 -- leaving him about 200 short of the 2,025 needed to clinch the nomination.

SUPERDELEGATES TO WEIGH IN?

But neither can win without help from superdelegates -- nearly 800 party insiders and officials who are free to back any candidate.

More than 250 superdelegates remain uncommitted. If the majority begin to move toward Obama they could quickly settle the race. Some Obama backers hoped the movement starts soon.

"It's now time for the superdelegates to begin bringing this process to a close," said Arizona Gov. Janet Napolitano, an Obama supporter.

Four superdelegates endorsed Obama on Wednesday, including George McGovern, the 1972 Democratic presidential nominee who switched his allegiance from Clinton to Obama.

"It's time for us to unite and get ready for the fall campaign," he told Reuters from his South Dakota home. He said he informed Clinton's husband, former President Bill Clinton.

"He just wanted me to know that he thinks that Hillary has made a great race and it's up to her to decide when she leaves. And I don't argue with that," McGovern said.

Another superdelegate, U.S. Rep. Heath Shuler of North Carolina, endorsed Clinton.

Obama, 46, took the day off at home in Chicago.

Sen. Claire McCaskill of Missouri, an Obama supporter, said Clinton should not be rushed out of the race. "I think it would be inappropriate, awkward and wrong for any of us to tell Sen. Clinton when it is time for the race to be over," she said.

Clinton won Indiana by fewer than 23,000 votes out of more than 1.25 million cast, taking the state by 51 percent to 49 percent. She had hoped to win by a bigger margin.

The 60-year-old former first lady still hopes to find a way to seat delegates from Michigan and Florida, where she won contests in January that are not recognized by the national party because of a dispute over their timing.

She said she would send a letter to Obama and party chairman Howard Dean saying the unrecognized elections were a civil and voting rights issue.

(Additional reporting by Jeff Mason, Steve Holland and Thomas Ferraro; editing by Chris Wilson)

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http:/blogs.reuters.com/trail08/ )