Although Barack Obama appears to hold a prohibitive advantage over Hillary Clinton in the Democratic presidential nomination race, one issue may delay the final coronation: Some Washington lawmakers are hesitating to announce their support for Sen. Obama, fearing a backlash from voters at home.

Sen. Obama appeared on Capitol Hill and visited the House floor Thursday, drawing a crowd. He pushed Democratic lawmakers to line up behind his campaign, but many fence-sitters aren't budging yet.

Sen. Obama leads Sen. Clinton in pledged delegates, popular vote and campaign cash. But the New York senator holds a 13-delegate lead among superdelegates, a category that includes party operatives, governors and members of Congress who can vote at the party's August convention.

About 250 superdelegates have yet to take sides, of whom about 90 are Washington politicians. Most of these politicians are themselves running in the fall.

Although Sen. Obama has picked up the endorsement of about 150 superdelegates since early February, many of whom are sitting politicians, recently he has had better luck among nonelected superdelegates. They don't risk the ire of constituents who might have favored Sen. Clinton or another candidate.

Many of the politicians sitting on the fence are from conservative states or mostly white, rural districts, where Sen. Obama has had the least success.

Louisiana Sen. Mary Landrieu, a Democrat, is expected to face a tough re-election fight in the fall and hasn't endorsed a presidential candidate. Despite her neutrality, the Republican Party has been airing an advertisement on YouTube that poses the question of whether she will endorse Sen. Obama. The ad transposes her picture with his picture and overlays it with a muddy recording of a speech in which he described small-town and working-class voters as "bitter."

A fellow Louisiana Democrat who recently won a special election, Rep. Don Cazayoux, was also the subject of Republican advertising that attempted to tie him to Sen. Obama. Rep. Cazayoux, a superdelegate, has declined to pick sides.

Sen. Landrieu hails from New Orleans, which went heavily for Sen. Obama in primary voting. She has received $10,000 from the Illinois senator's political action committee, federal records show. Nonetheless, she recently told reporters not to hold their breath waiting for her to decide, saying that neither candidate holds an edge among her constituency.

North Dakota Rep. Earl Pomeroy was one of 15 to 20 Democratic lawmakers who huddled with Sen. Obama Thursday in a townhouse on Capitol Hill for a meeting of undecided House members, most of them fiscal conservatives known as "Blue Dog Democrats." Mr. Pomeroy is backing Sen. Obama.

"He didn't have a big sales pitch -- he just wanted to give us a chance to get to know him a little better in an informal setting," Rep. Pomeroy said. He said the race is over for Sen. Clinton. "I think you kind of see resignation setting in with her supporters" in the House, he said.

Campaigning in West Virginia Thursday, Sen. Clinton repeated some of the key arguments she is making for continuing her bid, including that she is more electable against likely Republican nominee John McCain.

"Now the delegate math may be complicated, but the electoral math is easy," Sen. Clinton told a crowd in Charleston, W.Va. "We need 270 electoral votes to win in November." A poll this week suggested that Sen. Clinton would make a stronger match against Sen. McCain among West Virginia Democrats and independents.

Superdelegates hoping to avoid making a commitment have at least two possible ways out. One is if Sen. Clinton drops out of the race, leaving Sen. Obama as the nominee by acclamation. Another possibility is that superdelegates might endorse Sen. Obama en masse once the primary season is over, so that no single endorsement would stand out.

Sen. Obama played down the coming primaries in West Virginia and Kentucky, where Sen. Clinton is likely to win. "I think there's no doubt that she's the heavy favorite in both states," he said. "But my goal has been to spend time in all 50 states."

Oregon, Montana, South Dakota and Puerto Rico also will hold nominating elections in the next month.

Although Sen. Obama has said little about Sen. Clinton's refusal to concede the nominating contest, it is clear he is beginning to position himself as the nominee, looking toward the general election. Thursday, his campaign sent out a release outlining what it called "Vote for Change," a 50-state drive to register Democratic voters.