A new New York Times/CBS News poll also shows that Obama's support among Democratic voters nationally has softened over the last month, though he was supported by 46% of those surveyed, versus 43% for Clinton.
Obama is winning over superdelegates because "his arguments are more persuasive," said Mark Mellman, a Democratic pollster who is unaffiliated in the presidential race. "She obviously hopes that's going to change with Pennsylvania and races down the road. But for now, his arguments are being more persuasive with those superdelegates."
A major objective of Clinton's superdelegate operation is keeping supporters from defecting. Working from her campaign headquarters, a team of aides stays in seemingly constant touch with superdelegates committed to Clinton, sending them poll numbers and news articles meant to keep them from bolting.
"It's a slow drip, drip, drip -- but it's dripping the wrong way," said Joe Trippi, who was an advisor to former Democratic candidate John Edwards. "Psychologically, they're playing defense with superdelegates, not offense."
Some superdelegates in Clinton's camp are suggesting they might reconsider if she cannot meet certain goals, such as overcoming Obama's lead in the popular vote total. With 10 contests remaining, Obama has won about 700,000 more votes than Clinton. That tally excludes the votes in Florida and Michigan, which are not being recognized by the national Democratic Party.
Clinton aides would prefer that superdelegates consider a broader set of criteria, such as which candidate is likely to be more electable, or who ran more strongly in pivotal states such as Florida and Ohio.
Hoping that message will sink in, top aides hold regular conference calls with reporters in which a recurring theme is that superdelegates should see Clinton as the most formidable general-election candidate.
"The states she has taken have considerably more electoral votes," said Mark Penn, the campaign's chief strategist.
It is not clear that argument is resonating.
Rep. Lynn Woolsey (D-Petaluma), a leader of the influential House Out of Iraq Caucus, endorsed Clinton after hearing the New York senator explain her commitment to withdrawing U.S. forces from Iraq.
But last month, Woolsey began to adjust her position, committing herself to back the candidate with the bigger share of the popular vote.
"No one wants our party's nominee to be chosen by the votes of a handful of superdelegates," Woolsey said in a statement. "That's why, while I remain a strong Hillary Clinton supporter, I will cast my vote at the convention for the candidate that is chosen not through back-room deals, but by the votes of the American public."
New Jersey Gov. Jon Corzine, a superdelegate backing Clinton, gave a television interview Thursday in which he too said his support might hinge on the popular vote total.
Corzine was asked if a candidate could capture the nomination despite trailing in the popular vote. "I think it would be a very hard argument to make," the governor said.
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peter.nicholas@latimes.com
Times staff writers Noam N. Levey and Janet Hook contributed to this report.
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