WASHINGTON (AP) -- Democrats Hillary Rodham Clinton and Barack Obama headed into the 17th week of their increasingly contentious struggle for the party's presidential nomination on Sunday under widening fissures over the race, gender and economic status of U.S. voters.
As the candidates looked toward elections next week in North Carolina and Indiana, those issues remained in stark relief. Obama had a sizable lead in pre-balloting polls in North Carolina, where there is a large African American population, but he was running about even in with Clinton in Indiana.
His support there again was heavily dependent on urban areas with larger Black populations in contrast with Clinton's deeper support in rural- and small-town Indiana and among hard-pressed working class Whites -- especially women -- in rust-belt cities.
Those same divisions gave Clinton a nearly 10-percentage point victory last week in Pennsylvania, a vote that buried any speculation she would pull out of the race, where Obama holds an unassailable lead in elected delegates to the party's August national convention in Denver.
Obama said Sunday he needed to do more to raise his profile among working-class voters.
"I've got to be more present. I've got to be knocking on more doors. I've got to be hitting more events. We've got to work harder because although it's flipped a little bit, we've always been the underdog in this race," he said on Fox News Sunday.
"I'm running against the best brand in Democratic politics," he said.
There was increasing concern in the Democratic hierarchy that the Obama-Clinton struggle was undermining the party's chances against Arizona Sen. John McCain, the presumptive Republican nominee. He wrapped up the nomination two months ago and has used that time to define himself and unify his party behind him.
Against that background, however, there appeared little readiness -- especially in the Clinton camp -- to lessen the tensions among Democrats.
In fact, Obama and Clinton were hardening their rhetoric, with the former first lady challenging the Illinois senator to a one-on-one debate. He declined and retorted that the campaign has been reduced to trivia even as American working families suffer.
Clinton took the debate dispute to a new level, challenging Obama to face off with her in a debate without a moderator.
"Just the two of us, going for 90 minutes, asking and answering questions, we'll set whatever rules seem fair," she said Saturday while campaigning in South Bend, Indiana.
Obama refused the challenge Sunday in his appearance on Fox.
"I'm not ducking. We've had 21" debates, Obama said. "For two weeks, two big states, we want to make sure we're talking to as many voters on the ground, taking questions from voters," he said. "We're not going to have debates between now and Indiana."
Trailing in delegates and the popular vote, Clinton has been stepping up the pressure on Obama. She has argued that Obama won't debate because he's unhappy with questions from TV moderators during the April 16 debate just before the Pennsylvania vote. He has complained it focused too much on political trivia and too little on real issues.
At a town hall meeting in the aging industrial city of Anderson, Indiana, on Saturday, Obama said: "If you watched the last few weeks of this campaign, you'd think that all politics is about is negative ads and bickering and arguing, gaffes and sideline issues. There's no serious discussion about how to bring jobs back, to Anderson."
Clinton stumped the state with popular Sen. Evan Bayh, vowing to bring an industrial revival.
"We can do that again, but we need, as Senator Bayh said, a president who doesn't just talk about it but who actually rolls up her sleeves and gets to work," said Clinton.
They are after Indiana's 72 delegates and North Carolina's 115, when voters in both states vote May 6.
In the overall race for the Democratic Party nomination, Obama leads with 1,724.5 delegates, including superdelegates -- unelected party officials who can vote as they please. Clinton had 1,593.5, according to an Associated Press tally. It takes 2,025 to win the nomination.