"Please think through this decision," Clinton said, laughing and emphasizing the word "please."

"It is not a wise decision for yourself or your country."

The crowd applauded loudly.

A Gallup poll released this week indicated that 28 percent of Clinton's supporters would back McCain should the New York senator lose her quest for the Democratic nomination.

That compares to the 19 percent of Obama supporters who say they will favor McCain should Clinton be the party's nominee.

"First of all, every time you have a vigorous contest like we are having in this primary election people get intense," she continued. "You know, Sen. Obama has intense support. I have intense support."

Clinton stressed that there are "significant" differences between her and Obama, but said "those differences pale to the differences between us and Sen. McCain."

"I intend to do everything I can to make sure we have a unified Democratic party," she said. "When this contest is over and we have a nominee, we're going to close ranks, we're going to be united."