(CNN) -- Sen. Barack Obama took the lead in the race for superdelegates on the eve of a contest that's expected to fall easily into Sen. Hillary Clinton's column.

Sen. Barack Obama has a big lead over Clinton in overall delegates.

Rep. Tom Allen of Maine, Dolly Strazar of Hawaii, Sen. Daniel Akaka of Hawaii and Keith Roark of Idaho all endorsed Obama Monday, giving him a lead of four superdelegates for the time being.

Obama and Clinton face off Tuesday in West Virginia, where polls show Clinton ahead by more than a 40-point margin.

Under pressure from some to withdraw from the race, Clinton insists that West Virginia, where only 28 delegates are at stake, is a key state in the fight for the White House.

She said again Monday that no Democratic candidate since 1916 has gone on to win the White House without first winning West Virginia.

"West Virginia is making a decision that has far-reaching consequences to send a message to people what you expect from your next president," she said at a stop in Clear Fork, West Virginia.

Clinton currently trails Obama across all fronts -- superdelegates, pledged delegates and the popular vote, according to CNN's latest estimates.

Obama leads in the race for superdelegates, 277 to Clinton's 273, and he's ahead in the overall delegate count, 1,869 to 1,697.

At the beginning of the year, Clinton led the superdelegate race by more than 100.

Clinton has vowed to stay in the race until someone gets enough delegates to clinch the nomination.

The focus of the Democratic race has largely turned to the superdelegates because they outnumber the remaining pledged delegates up for grabs.

Superdelegates are party leaders and officials who will vote for the candidate of their choice at the Democratic convention in August. Some have already committed to vote for a particular candidate and some have not.

Clinton's campaign says she would be more electable in a general election because she has done well in swing states like Ohio and Pennsylvania, as well as Florida and Michigan, which were stripped of their delegates.

West Virginia is also a key swing state. Bill Clinton won in 1992 and 1996, and George Bush carried it in 2000 and 2004.

Obama spoke in Charleston, West Virginia, on Monday, sounding patriotic themes and saying he expected Clinton to win on Tuesday.

"But when it's over, what will unify us as Democrats -- what must unify us as Americans -- is an unyielding commitment to the men and women who've served this nation and an unshakable fidelity to the ideals for which they've risked their lives," Obama said.

Clinton's campaign says if she leads in the popular vote, she should become the Democratic nominee.

"Hillary is within striking distance of winning the popular vote nationwide -- a key part of our plan to win the nomination," campaign chairman Terry McAuliffe said in a letter to supporters Sunday.

Her campaign is trying to turn out the vote in the remaining six contests, hoping the popular vote argument will persuade superdelegates to endorse her instead of Obama.

West Virginia Gov. Joe Manchin, an uncommitted superdelegate, said the delegate numbers are in Obama's favor, but the popular vote is important to the people of his state. Watch what Manchin says about Tuesday's contest »

"I think we see what happened in 2000, when Al Gore won the popular vote, and where the country has gone and the feelings toward government since then. I put a lot of stock in that," he said on CNN's "American Morning."

"If the people believed that it was over, they wouldn't be voting maybe in the way they might vote tomorrow or in the next few campaigns," he said.

Clinton is expected to trounce Obama in West Virginia, but Manchin said he thinks Obama would also be able to carry the state in the general election.

Clinton had a 43-percentage-point advantage over Obama in West Virginia, 66 percent to 23 percent, according to a survey from the American Research Group released Friday.

The poll was conducted after last Tuesday's contests and carries a margin of error of plus or minus 4 percentage points.

"This state is really Hillary Clinton's wheelhouse. It's an older population, socially conservative, blue-collar workers," said Kennie Bass, a political reporter for WCHS in West Virginia.

CNN's Alexander Marquardt contributed to this report.

All About U.S. Presidential Election • West Virginia