Obama, an Illinois senator, leads the Democratic race in the number of delegates, the popular vote and in many national opinion polls. Clinton, D-N.Y., on the other hand, vowed to fight on as Obama politely acquiesces, The Philadelphia Inquirer said Sunday.

Obama campaign insiders said if the calls for Clinton to bow out continue beyond a few days, the tide may shift in Obama's favor among Democratic voters in Pennsylvania.

Several leading Democrats, including Sen. Patrick Leahy of Vermont, said the Democratic contest is growing tedious and have called for Clinton to abandon her bid for president.

Obama has called the Democratic contest a healthy sign of democracy, but joked the fight for the nomination has gone on so long that children born when the race started can walk and talk by now, the Inquirer said.