Last night’s presidential candidate forum in Chicago signals an increasingly bitter fight for the Democratic nomination, with the leading contenders, Senators Clinton and Obama, struggling to fend off attacks from rivals jostling for the spotlight in a crowded primary field.

Appearing before an estimated 15,000 union members at Chicago’s Soldier Field in a televised outdoor forum, seven candidates sparred over foreign policy, trade, and other issues as they courted the endorsement of key labor unions within the AFL–CIO.

With a widening lead in national polls and facing criticism for her support of lobbyists, a frequently smiling Mrs. Clinton seemed to relish her position as the front-runner even as she tried to take the high road in disputes over ethics and foreign policy.

While at one point saying she did not want to “get in fights with Democrats,” she later lectured Mr. Obama over his recent statements suggesting he would be willing to order a unilateral strike on terrorist targets in Pakistan if necessary. Treating the first-term Illinois senator like a newcomer to the world stage — and hearing some boos from the raucous stadium crowd in the process, Mrs. Clinton said it would be a “very big mistake” to telegraph military decisions that could destabilize an allied regime and told Mr. Obama to “remember” that Pakistan has nuclear weapons.

“You can think big, but remember you shouldn’t always say everything you think if you’re running for president, because it has consequences across the world,” Mrs. Clinton said. “And we don’t need that right now.”

Mr. Obama faced the same criticism in an extended exchange with Senator Dodd of Connecticut, who told him “was wrong” to threaten Pakistan. Mr. Obama responded with a criticism of Mr. Dodd and the other candidates, Mrs. Clinton included, who voted to authorize the Iraq war. “I find it amusing that those who helped to authorize and engineer the biggest foreign policy disaster in our generation are now criticizing me for making sure that we are on the right battlefield and not the wrong battlefield in the war against terrorism,” he said.

As candidates vied for the support of the country’s largest federation of labor unions, Mrs. Clinton came under fire for her ties to the business community.

In a bid to portray the former first lady as a friend of corporations, John Edwards took aim at a recent issue of Fortune magazine that featured a flattering profile of Mrs. Clinton.

“The one thing you can count on is you will never see a picture of me on the front of Fortune magazine saying, ‘I am the candidate that big, corporate America is betting on,'” the former North Carolina senator said. “That will never happen. You can take that to the bank.”

Struggling at first to respond, Mrs. Clinton flashed a grin. “I’m just taking it all in,” she said, adding she had noticed in recent days “that a lot of the other campaigns have been using my name a lot.”

She said she wanted “a united Democratic Party” before turning to her experience as the favorite target of Republicans. “For 15 years, I have stood up against the right-wing machine, and I’ve come out stronger,” she said, drawing applause. “So if you want a winner who knows how to take them on, I’m your girl.”

Without mentioning Mrs. Clinton by name, Messrs. Obama and Edwards sought repeatedly to cast her as a “Washington insider” ill-equipped to stand up to entrenched interest groups in the name of reform. They each reminded viewers of their pledges not to accept donations from Washington lobbyists or political action committees.

In a debate last weekend before left-leaning Democratic bloggers, Mrs. Clinton defended lobbyists, saying they represented “real people.”

The dispute over ethics also spilled into key union issues such as trade, where Messrs. Obama and Edwards said they would be more effectively in negotiating agreements that included higher labor standards.

“The question is: On whose behalf is the president negotiating?” Mr. Obama said. “Is he — or she — negotiating on behalf of the people in this stadium or are you only negotiating on behalf of corporate profits?”

“That is an important issue, and it is an important distinction that we have to make,” he added.

All of the candidates voiced criticism of the North American Free Trade Agreement, signed by President Clinton, but only Rep. Dennis Kucinich of Ohio said he would scrap it entirely.

The labor forum came as the executive board of the AFL–CIO is set to meet today to decide whether to endorse a candidate. The organization’s president, John Sweeney, described it as “one giant job interview.” The umbrella union, which represents more than 10 million Americans nationwide, was not expected to throw its considerable backing behind one contender, instead leaving its individual unions free to endorse on their own.

Though membership has declined across the country in recent decades, unions remain one of the Democratic Party’s key constituencies. The leading White House contenders have courted them aggressively, pitching themselves as longtime champions of the labor movement.

A former Alaska senator, Mike Gravel, did not participate in last night’s forum, which was broadcast by MSNBC.