PHILADELPHIA — Perhaps the analogy was inevitable: Hillary Rodham Clinton as Rocky Balboa, the scrappy underdog boxer from Philadelphia memorably depicted in the 1976 Oscar-winning film. Even if Rocky did lose his first big fight.

Addressing a meeting of the Pennsylvania AFL-CIO Tuesday, the former first lady and New York senator said she, like Rocky, wasn't a quitter.

Recalling a famous scene on the steps of the Philadelphia Museum of Art, Clinton said to end her presidential campaign now would be as if "Rocky Balboa had gotten halfway up those art museum steps and said, 'Well, I guess that's about far enough.'"

"Let me tell you something, when it comes to finishing a fight, Rocky and I have a lot in common. I never quit. I never give up. And neither do the American people," Clinton said.

She promised that as president she would create 3 million new jobs through investments in public infrastructure.

Clinton also told the labor audience that as first lady she had forcefully battled NAFTA, the North American Free Trade Agreement, even as her husband was aggressively pushing for its passage through Congress. The agreement is widely unpopular with organized labor because it helped clear the way for many blue-collar jobs to be moved to Mexico and other countries with cheaper labor costs.

"I did speak out and oppose NAFTA," she said. "I raised a big yellow flag and said 'I don't think this will work.'"

How strongly Clinton worked against NAFTA while in the White House remains a matter of some dispute. Former aides to Bill Clinton have said she was skeptical of the agreement, but largely because she felt it conflicted with her effort to pass health care reform.

Speaking to reporters later, Clinton insisted she had voiced objections to the substance of the proposal, not just its timing.

"I was in many meetings starting in the '92 campaign _ I raised questions," she said. "I did it in the White House again, in meetings with as many different audiences in the White House in the decision-making process that I could speak to. But the president made a decision. As part of an administration, I believe you support the president, and I did."

Clinton also warned the labor audience that Democrats won't have an easy time against Republican nominee-in-waiting John McCain in the general election, and implied that Sen. Barack Obama of Illinois, her rival for the Democratic nomination, may not be up to the task.

"The Republicans aren't going to give up without a fight," Clinton said. "And no matter how beautiful your rhetoric, the Republicans aren't going to turn off their attack machine _ it doesn't have an off-switch."

"But one thing you know about me is that when I say I'll fight for you, I'll fight for you," she said. "I know what it's like to stumble. I know what it means to get knocked down. But I've never stayed down, and I never will."

The fighter theme is a recurrent one for Clinton and is being carried in a new pro-Clinton radio ad by the American Federation of Teachers airing in Pennsylvania. In the 60-second ad, a supporter says: "She stands up for what she believes. When she gets knocked down she get up and keeps on fighting."

The teachers' union has spent about $1.9 million on ads and direct mail supporting Clinton in primary and caucus states since December. The Pennsylvania ad began airing Monday and will run through April 21 at a cost of $329,000. The Pennsylvania primary is April 22.

In recent days, Clinton has made an issue of calls from some leading Democrats for her to abandon the race. Vermont Sen. Patrick Leahy, a prominent Obama backer, last week called on her to step aside, arguing that she was never going to win enough delegates and suggesting that she should bow out in "the interests of a Democratic victory in November."

"Now, this is one of the most important elections we've ever had," Clinton said Tuesday. "There is so much at stake. But just as it's getting time to vote here in Pennsylvania, Senator Obama says he's getting tired of it. His supporters say they want it to end."

Clinton's campaign manager, Maggie Williams, on Tuesday compared calls for Clinton to end her campaign to the Florida recount in 2000, a still bitter episode for many Democrats

"The last time that we were told we'd better cut the process short or the sky would fall was when the Supreme Court stopped the Florida recount in 2000," Williams said in a widely distributed campaign memorandum. "But Chicken Little was wrong. What was true then is true now: There is nothing to fear and everything to gain from hearing from all of the voters."

Obama disputed the notion that his backers are behind a drive to get Clinton to drop out.

"You know we had one supporter, Pat Leahy, say something and they've been, you know, working that pretty hard for the last week now," Obama told Pittsburgh radio station KDKA. "I've said for the last three days that I think that Senator Clinton should stay in the race as long as she wants. ... She has every right to compete and I'm looking forward to competing against her."