Mr. Obama cautiously walked through a long answer that ended with a plan to give legal status to long-established undocumented immigrants. "There. Another question," he said, shutting down discussion.

The debate over how to deal with undocumented immigrants split the Republican Party two years ago, infuriating its social-conservative base and driving away Hispanic voters. It could be even more perilous for Democrats.

Democratic strategists believe Hispanic voters could swing a decisive handful of states — including Florida, Arizona, New Mexico, Colorado and Nevada — to the Democrats in 2008, ensuring the election of a Democratic president and cementing a Democratic majority for years to come. But the party's blue-collar, middle-income and African-American supporters are increasingly angry about undocumented immigration, much of it Hispanic.

Democrats "are pretty jumpy on the issue," says Rep. Luis Gutierrez, an Illinois Democrat who pushed for immigration overhaul in the House. "They would prefer to allow the Republicans to shepherd the Hispanic votes into the Democratic column without having to scare away a single other voter themselves," he says.

"That's not likely to happen. "This election could turn on this issue if we don't handle it intelligently," says Connecticut Sen. Christopher Dodd, a Democratic presidential candidate. After a recent Iowa City foreign-policy speech, four of the 30 questions passed up to him from the audience were about immigration.

In a Nov. 5 Wall Street Journal/NBC News poll, 11% of adults — and 4% of Democratic voters — said undocumented immigration is their top priority. But members of that minority, organized on the Internet, have created political turmoil by flooding lawmakers' offices with faxes and regularly raising the issue on the campaign trail.

Similarly, a November University of Iowa poll shows just 2.4% of Iowa Democrats consider immigration as the issue "most important" to determining their vote, but 85% said a candidate's position on immigration is important or very important to them.

In one sign of the tension within the Democratic caucus, Hispanic-American lawmakers were furious last week that Democratic leaders hadn't derailed Republican efforts to include a limited English-only measure in a budget bill.

Hispanics made up 8% of the national vote in 2006, but their growing numbers and anger with the Republicans over such talk could mean electoral gold for the Democrats. NDN, a nonprofit Democratic think tank, predicts "there is no reasonable Republican road map to victory in 2008" if growing Hispanic populations tip several key states into the Democratic column.

But a pro-immigration policy risks alienating other Democratic constituencies. Rep. Gutierrez blames the weakening U.S. economy for fanning immigration anger among working-class voters. "It's easy because people are afraid" about wages, mortgages and jobs, he says.

Even here in Chariton, where the Census Bureau reports there are only 40 foreign-born residents, the Obama audience worried that Mexican workers would displace Americans. "Joe Blow Citizen knows what's going on," a middle-aged man told Mr. Obama. Schools crowded with Spanish-speaking children and the lawlessness of undocumented immigration seem to anger campaign audiences almost as much.

"A heck of a lot of middle-class Democrats feel they're being overwhelmed by undocumented immigrants and they're reacting the same as Republicans, only they're more ashamed to say so," says University of Virginia political scientist James Ceaser.

Democrats also risk setting off a "rivalry between the minorities" if they tilt toward Hispanics with their immigration policy, says the University of Virginia's Mr. Ceaser. The rise of Hispanic political power has come largely at the expense of African Americans, and Hispanic immigrants have largely replaced blacks in some industries, including construction.

Miami Urban League president T. Willard Fair, among the few black leaders who speak openly about the effects of undocumented immigration on African-Americans, says "there is an undercurrent" of resentment in his community. Blacks aren't likely to leave the Democratic party over it, he says, and they're reluctant to publicly oppose immigration because of their own civil rights history. But "all you have to do is take a walk to the neighborhood bar to hear the talk," he says.

The Democrats' problem is that they raised immigration to a national issue by promoting an overhaul plan. Then, while it was left bubbling, they failed to pass a bill, despite having a majority in Congress, says Democratic strategist Peter Brodnitz who isn't affiliated with a presidential candidate.

Democrats further angered many voters by proposing some benefits for undocumented immigrants before producing any economic relief for worried middle-class voters, adds Mr. Brodnitz.

Polls regularly show a majority of Americans resigned to legalizing immigrants if the process includes penalties and is balanced by improved border and workplace enforcement. But polls also show that Americans are outraged about government benefits for undocumented immigrants, which makes framing a campaign message tricky for the Democrats.

In a new Quinnipiac University poll of voters in Ohio, a toss-up state, 55% favored legalizing unlawful immigrants. But only 11% said they should be allowed a driver's license and just 35% said their children should be allowed to attend public school.

Republicans, meanwhile, see undocumented immigration as a campaign bonanza because it motivates their base voters while diverting attention from the Iraq war and sowing discord among the Democrats. In the Wall Street Journal/NBC poll, 38% of Republicans said undocumented immigration is their first or second priority.

That has turned immigration from a one-time fringe issue into mainstream politics. Former New York Mayor Rudy Giuliani and former Massachusetts Gov. Mitt Romney, who while they were in office advocated plans to help legalize unlawful immigrants, now pound away at plans to deny them jobs and benefits.