Those surveyed are inclined to fear that the massive legislation will increase their costs and hurt the quality of health care their families receive, although they are more positive about its impact on the nation's health care system overall.

Supporters "are not only going to have to focus on implementing this kind of major reform," says Robert Blendon, a professor of health policy and political analysis at Harvard. "They're going to have to spend substantial time convincing people of the concrete benefits of this legislation."

The risk for them is that continued opposition will fuel calls for repeal and dog Democrats in November's congressional elections. The bill was enacted without a single Republican vote.

In an interview airing Tuesday on NBC's Today, Obama acknowledges concerns about cost. "It is a critical first step in making a health care system that works for all Americans," he said of the law, adding, "We are still going to have adjustments that have to be made to further reduce costs."

Obama's approval rating was 47%-50% — the first time his disapproval rating has hit 50%.

In the survey:

• A plurality predicts the law will improve health care coverage generally and the overall health of Americans. But a majority says it also will drive up overall costs and worsen the federal budget deficit.

• When it comes to their families, they see less gain and more pain: Pluralities say it will make coverage and quality of care worse for them. By 50%-21%, they predict it will make their costs higher.

Opponents of the health care bill are a bit more likely than supporters to say the vote will have a major impact on their vote for Congress in the fall. Three in 10 are much more likely to vote for a candidate who opposes the bill. One in four are much more likely to vote for a candidate who supports it.

The poll of 1,033 adults, taken by land line and cellphone Friday through Sunday, has a margin of error of +/–4 percentage points.

Half call passage of the bill "a bad thing" and 47% "a good thing." That differs from a one-day USA TODAY poll taken March 22 — a day after the House approved the legislation — in which a 49%-40% plurality called the bill "a good thing."

"Any one-day poll in the immediate aftermath of a major event is likely to be subject not only to sampling error but also to very short-term effects," says political scientist Charles Franklin of the University of Wisconsin-Madison. At the time, "the news cycle was dominated by the positive side of the story, and only a little bit by the Republicans' rebuttal to that."

There was a strong reaction against the tactics Democratic leaders used to pass the bill. A 53% majority call Democratic methods "an abuse of power;" 40% say they are appropriate.

And when asked about incidents of vandalism and threats that followed the bill's passage, Americans are more inclined to blame Democratic political tactics than critics' harsh rhetoric. Forty-nine percent say Democratic tactics are "a major reason" for the incidents, while 46% blame criticism by conservative commentators and 43% the criticism of Republican leaders.