Barack Obama's sprint to the Democratic nomination received another boost Friday as two more superdelegates pledged their support, including one who dropped his backing for Hillary Rodham Clinton's faltering White House bid.

The two Democrats have been lobbying superdelegates _ who are not bound by state results _ to line up behind them in the final push for the nomination. While Obama has presented himself, albeit subtly, as the inevitable candidate, Clinton's efforts have been to ward off further defections and convince the crucial voting block that her candidacy still retained signs of life.

Obama, a step closer to making history after a strong showing in two state primaries earlier this week, appeared to be more convincing in delivering his message.

On Friday, he picked up two new superdelegates, including one who had previously supported the former first lady, Obama spokesman Dan Pfeiffer said. He had received the backing of two others Thursday, an announcement that came shortly after he made an impromptu, and well-received, appearance in the U.S. House of Representatives.

Clinton, who had also earlier made a similar pitch in Congress, campaigned on friendly turf in West Virginia, where she is expected to win in the next vote Tuesday. She told supporters she had faced similar pressure to withdraw before she went on to win New Hampshire, Ohio, Texas and Pennsylvania's primaries.

"I'm running to be president of all 50 states," Clinton, who would be the nation's first female president, said Thursday. "I think we ought to keep this going so the people of West Virginia's voices are heard."

But that race _ one of six remaining contests _ is unlikely to decide a 16-month nomination battle that has both polarized and riveted Democratic voters. With 217 delegates yet to be awarded, it is mathematically impossible for either candidate to clinch the nomination.

As of late Thursday, Obama has 1,849.5 delegates to Clinton's 1,697, with 2,025 needed to win the nomination.

That means the roughly 800 superdelegates are the ones likely to decide the outcome. Of that total, about 260 remain uncommitted. About a third of the undeclared superdelegates are members of Congress.

On the cusp of making history with his bid to be the U.S.'s first black president, he gently nudged Congressional superdelegates for their support.

The first-term Illinois senator was surrounded in the House by well-wishers calling him "Mr. President" and reaching out to pat him on the back. The glad-handers included a few Republicans, as well as Clinton supporters.

But the 46-year-old was quick to note that he faced a formidable adversary.

"Our goal is going to be to try to be to bring the party together as soon as possible," Obama said after his visit. "But we still have contests remaining, and so in no way am I taking this for granted. We're going to have to keep on working."

"My main message is that whichever way you want to go, the sooner that superdelegates make their decision the sooner we will have a sense of who the nominee will be and sooner we can focus on John McCain," Obama told the Fox News Channel outside his Senate office.

Pfeiffer, Obama's spokesman, said candidate plans to receive the endorsement of Rep. Peter DeFazio of Oregon an event Friday in his homestate. Oregon holds its primary on May 20.

While polling in the Pacific Northwest state has been sparse, Obama is believed to hold a significant advantage over Clinton. With DeFazio, he will have the endorsement of three of the state's four Democratic House members.

The Associated Press has contacted nearly 100 of the undeclared superdelegates since the Tuesday contests and has found that many see Obama as the likely nominee, but are reluctant to make a public commitment until after the final states hold their votes June 3.

Obama has focused more intently on Republican John McCain in recent weeks.

On Thursday, he accused McCain in a CNN interview of "losing his bearings" for repeatedly suggesting the Islamic militant group Hamas preferred Obama for president.

That brought an angry response from McCain's campaign, which accused Obama of trying to make an issue of McCain's age. Age is a touchy subject for McCain, who turns 72 in August and would be the oldest person to be sworn in as president if elected.

Meanwhile, Clinton continued to press her case that she was the candidate most equipped to defeat McCain in November, though new comments about race dogged her following an interview with USA Today published Thursday.

In it, Clinton cited an Associated Press article which found that "Senator Obama's support among working, hardworking Americans, white Americans, is weakening again, and how whites in both states who had not completed college were supporting me."

"There's a pattern emerging here," she said.

Obama's campaign did not respond to the comments, which generated buzz in the liberal blogosphere.

Clinton has repeatedly vowed not to drop out before June. But as the race drags on, fears mount that it could affect party unity, costing Democrats their best chance to reclaim the White House after almost eight years of Republican leadership.