WASHINGTON -- Americans are worried about the fine print in the health care overhaul, an Associated Press poll says, and those concerns are creating new challenges for President Barack Obama as he tries to overcome doubts in Congress.

Despite a widely shared conviction that major health care changes are needed, Democratic bills that aim to extend coverage to the uninsured and hold down medical costs get no better than a lukewarm reception in the latest results.

The poll found that 43 percent of Americans oppose the health care plans being discussed in Congress, while 41 percent are in support. An additional 15 percent remain neutral or undecided.

There has been little change in that broad public sentiment about the overhaul plan from a 40-40 split in an AP poll last month, but not everyone's opinion is at the same intensity. Opponents have stronger feelings on the issue than do supporters. Seniors remain more skeptical than younger generations.

The latest survey was conducted by Stanford University with the nonprofit Robert Wood Johnson Foundation.

The United States is the only developed nation that does not have a comprehensive national health care plan for all its citizens, leaving about 50 million of America's roughly 300 million people uninsured. Obama, who took office in January, campaigned on a promise of offering affordable health care to all Americans.

When the AP poll questions were framed broadly, the answers seemed to indicate ample support for Obama's goals. When required trade-offs were brought into the equation, opinions shifted -- sometimes dramatically.

In one particularly striking finding, the poll indicated that public support for banning insurance practices that discriminate against those in poor health may not be as solid as it seems.

A ban on denial of coverage because of pre-existing medical problems has long been one of the most popular consumer protections in the health care debate. Some 82 percent said they favored the ban, according to a Pew Research Center poll in October.

In the AP poll, when told that such a ban would probably cause most people to pay more for their health insurance, 43 percent said they would still support doing away with pre-existing condition denials but 31 percent said they would oppose it.

Costs for those with coverage could go up because people in poor health who'd been shut out of the insurance pool would now be included, and they would get medical care they could not access before.

"I'm thinking we'd probably pay more because we would probably be paying for those that are not paying. So they got to get the money from somewhere. Basically I see our taxes going up," said Antoinette Gates, 57, of Atlanta.

The health care debate is full of such trade-offs. For example, limiting the premiums that insurance companies can charge 50-year-olds means that 20-year-olds have to pay more for coverage.

"These trade-offs really matter," says Robert Blendon, a professor at the Harvard School of Public Health who follows opinion trends. "The legislation contains a number of features that polls have shown to be popular, but support for the overall legislation is less than might be expected because people are worried there are details about these bills that could raise their families' costs."

If the added costs -- spread over tens of millions of people -- turn out to be small, it probably won't make much difference, Blendon said. But if they're significant, Obama could be on shaky ground in the final stretch of his drive to deliver access to health insurance to most Americans.

More than 4 in 5 Americans now have health insurance, and their perceptions about costs will be critical as Obama tries to close the deal. Democrats in the House came together to pass a bill, but in the Senate, Democratic liberals and moderates disagree on core questions.

The poll suggests the public is becoming more attuned to the fact that when it comes to health care, details often make all the difference.

For example, asked if everyone should be required to have at least some health insurance, 67 percent agreed and 27 percent said no.

The responses flipped when people were asked about requiring everybody to carry insurance or face a federal penalty: 64 percent said they would be opposed, while 28 percent favored that.

Both the House and Senate bills would require all Americans to get health insurance, either through an employer, a government program or by buying their own coverage. Subsidies would be provided for low-income people, as well as many middle-class households.

And there would also be a stick -- a penalty collected through the income tax system to enforce the coverage mandate.

"I think it's crazy. I think it infringes on our rights as a citizen, forcing us to do these things," said Eli Fuchs, 26, of Marietta, Georgia.

The poll was based on land line and cell phone interviews with 1,502 adults from Oct. 29 to Nov. 8. It has a margin of error of plus or minus 2.5 percentage points. The interviews were conducted by GfK Roper Public Affairs and Media. Stanford University's participation was made possible by a grant from the Robert Wood Johnson Foundation, a nonpartisan organization that conducts research on the health care system.