MILWAUKEE — Representative John Lewis, an elder statesman from the civil rights era and one of Senator Hillary Rodham Clinton’s most prominent black supporters, said Thursday night that he planned to cast his vote as a superdelegate for Senator Barack Obama in hopes of preventing a fight at the Democratic convention.
“In recent days, there is a sense of movement and a sense of spirit,” said Mr. Lewis, a Georgia Democrat who endorsed Mrs. Clinton last fall. “Something is happening in America, and people are prepared and ready to make that great leap.”
Mr. Lewis, who carries great influence among other members of Congress, disclosed his decision in an interview in which he said that as a superdelegate, he could not go against the wishes of the voters of his district, who overwhelmingly supported Mr. Obama.
“I’ve been very impressed with the campaign of Senator Obama,” Mr. Lewis said. “He’s getting better and better every single day.”
His comments came as fresh signs emerged that Mrs. Clinton’s support was beginning to erode from some other African-American lawmakers who also serve as superdelegates. Representative David Scott of Georgia, who was among the first to defect, said he, too, would not go against the will of voters in his district.
The developments came on a day in which Mrs. Clinton set out anew to prove that the fight for the Democratic nomination was far from over. Campaigning in Ohio, she pursued a new strategy of biting attack lines against Mr. Obama, while adopting a newly populist tone as she courted blue-collar voters.
Mrs. Clinton also intensified her efforts in Wisconsin, which holds its primary on Tuesday and where she and Mr. Obama now have the first dueling negative television advertisements of the campaign.
In the ads, Mrs. Clinton taunted Mr. Obama for refusing to debate her in Wisconsin. And she and former President Bill Clinton prepared for a new fund-raising blitz to try to counter Mr. Obama’s edge of several million dollars in campaign cash.
Yet even as the Democratic rivals looked ahead to the primaries in Wisconsin, Ohio and Texas, Mr. Lewis said he and other prominent African-American party leaders had been moved by Mr. Obama’s recent victories and his ability to transcend racial and geographic lines.
Though Mr. Lewis had praise for Mrs. Clinton and for her historic candidacy, he said he could decide within days whether to formally endorse Mr. Obama.
He also said he and other lawmakers would meet in the coming days to decide how they intended to weigh into the nominating fight. If neither Mrs. Clinton nor Mr. Obama receive enough pledged delegates to win the nomination, superdelegates like Mr. Lewis may play the deciding role in who wins.
“If I can be used as a mediator, a negotiator or a peacemaker, I’d be happy to step in,” Mr. Lewis said, adding that he intends to speak to both candidates in hopes of ending the race amicably in the next month. “I don’t want to see Mrs. Clinton damaged or Mr. Obama damaged.”
Jay Carson, a spokesman for Mrs. Clinton, said Thursday: “Congressman Lewis is a true American hero, and we have the utmost respect for him and understand the great pressure he faced. And Senator Clinton enjoys incredibly strong support from superdelegates around the country from all regions and races.”
The comments by Mr. Lewis underscored a growing sentiment among some of the party’s black leaders that they should not stand in the way of Mr. Obama’s historic quest for the nomination and should not go against the will of their constituents. As superdelegates, they may have the final say, which is something Mr. Lewis said he feared would weaken Democrats and raise Republicans’ chances of winning the White House.
Still, the Democratic nominating fight clearly has many turns ahead. On Thursday, Mrs. Clinton unleashed the most ambitious mobilization of her forces in weeks, reflecting the intense pressure she is under from Mr. Obama, the political necessity for her of towering performances in the delegate-rich primaries in Ohio and Texas on March 4, and her fresh hope of an upset victory in Wisconsin.
Specifically, Mrs. Clinton is hoping to gain political mileage by turning one of Mr. Obama’s attributes — his oratory — against him. She is warning voters about politicians who give great speeches and make big promises but ultimately do not deliver on them.
“Speeches don’t put food on the table,” Mrs. Clinton said at a General Motors plant in Warren, Ohio, on Thursday morning. “Speeches don’t fill up your tank, or fill your prescription, or do anything about that stack of bills that keeps you up at night.”
“My opponent gives speeches,” she added. “I offer solutions.”
Mrs. Clinton has been also criticizing Mr. Obama with populist language, saying she would “take on” insurers and credit card companies and “go after” drug companies. She portrayed Mr. Obama as untested on the battlefield against special interests.
If there was a sign of the imbalance in momentum between Mrs. Clinton and Mr. Obama on Thursday, it could be gleaned from Mr. Obama’s travel itinerary. He took a respite from the campaign trail, aides said, so he could spend Valentine’s Day with his family in Chicago before returning to Wisconsin on Friday.
Clinton advisers said Thursday that it was unlikely they would broadcast “horrible nasty negative ads,” in the words of one adviser, and that they were wary of going too negative against Mr. Obama, given what the Clintons say is the news media’s tendency to coddle and protect Mr. Obama and portray the Clintons as an attack machine.
At the same time, Clinton advisers say that the stakes are so high — in Ohio and Texas in particular — that Mrs. Clinton cannot afford to let Mr. Obama gain momentum. In Wisconsin, for instance, Mrs. Clinton is hoping to stave off a blowout — and perhaps even pull off a surprise — by blasting Mr. Obama for refusing to debate her there.
“The last time we debated was in California, and I convincingly won California, which may be why Senator Obama doesn’t want to have a debate in Wisconsin,” Mrs. Clinton said in a telephone conference call with reporters.
Mr. Carson, her spokesman, said she would keep the debate issue alive until Tuesday.
“A refusal to debate one’s primary opponent is always seen by regular voters as being chicken,” he said. “And voters, especially Democratic voters hungry for a general election win, want a candidate who is tough and ready.”
Mr. Obama responded to the attacks with a television spot of his own in Wisconsin.
“After 18 debates, with two more coming, Hillary says Barack Obama is ducking debates?” the advertisement says, showing images from their debates over the last year. “It’s the same old politics, of phony charges and false attacks.”
As Mrs. Clinton was delivering her criticism of Mr. Obama in Ohio, a similar argument was presented to Wisconsin voters by Mr. Clinton, who referred to Mr. Obama as “the excitement of the now.”
“It’s about whether you choose the power of solutions over the power of speeches,” Mr. Clinton told a small gathering of voters in Milwaukee, ticking through a list of his wife’s platforms and accomplishments.
In New Mexico, one of the more than 20 states to hold contests on Feb. 5, the votes were finally counted Thursday, giving Mrs. Clinton a victory and providing more evidence that the contest was far from concluded. She continued to hold a lead among superdelegates, even as a New Jersey official, Christine Samuels, changed her support to Mr. Obama and at least two others went back to being uncommitted.