Through March 1, 4.2 million Americans had enrolled in health plans via the Affordable Care Act's insurance exchanges, the government said this week. The deadline for enrollment is March 31, and the Congressional Budget Office has estimated that 6 million people will sign up this year for private plans

Highest acceptance

Fifty-one percent of Americans favor retaining the Affordable Care Act with "small modifications," while 13 percent would leave the law intact and 34 percent would repeal it. That's the highest level of public acceptance for the law yet in the Bloomberg poll.

Investors are betting the law will withstand political challenges. An "Obamacare" portfolio of stocks that benefit from the law developed by the online broker Motif Investing is up 40.9 percent over a year ago as of March 12, almost doubling the performance of the Standard & Poor's 500 index, which returned 22.9 percent.

A "Repeal Obamacare" portfolio underperformed the benchmark stock index, rising 16.1 percent during the period.

The law's opponents have the advantage of intensity, which was on display in the March 11 Florida election for a vacant congressional seat. After a campaign focused on differences over Obamacare, Republican David Jolly turned out more of his supporters than did Democratic candidate Alex Sink. The election drew little more than half as many voters in the district as in the 2012 presidential race, when Obama narrowly carried it and a since-deceased Republican congressman was re-elected.

'Major' decider

"In off-year elections, turnout is a huge factor," said J. Ann Selzer, who conducted the survey for Bloomberg. "The anti-Obamacare segment is both more likely to say they will definitely vote and more likely to say their vote will be strongly influenced by their view of Obamacare; that can be enough to sway a race."

Seventy-three percent of Bloomberg poll respondents who would repeal Obamacare say the law will be a "major" decider of their vote, compared with 45 percent of those who support modifications and 33 percent of those who back the law as is.

Repeal advocates are also the most likely to vote, with 73 percent saying they will "definitely" do so. By contrast, 61 percent of those who want only small modifications are likely voters as are 54 percent of those who want the law kept intact.

Alternate perceptions

Republicans and Democrats have become so polarized over the Affordable Care Act that they have alternative perceptions of how the law has touched their families and friends.

Fifty-nine percent of Republicans say they personally know someone who has been hurt by the law and only 14 percent say they know someone who has been helped. Among Democrats: 48 percent say they know someone the law has helped and only 19 percent know anyone who has been hurt.

Party identification and political ideology track responses to the question more closely than do traits such as income, education and race that usually are more closely linked to differences in health-coverage experiences.

Even with public acceptance of the law, 72 percent of Republicans favor repeal. That's one reason the Republican- controlled U.S. House has voted about 50 times to repeal all or part of the law and opposition is an article of faith among the party's presidential aspirants.

Keeping provisions

Still, rank-and-file Republicans want several key provisions retained. Sixty-two percent of Republicans want to retain the law's ban on denying coverage based on pre-existing medical conditions, and 57 percent want to keep the requirement that insurance companies allow children up to age 26 to stay on their parents' policies.

Republicans are about evenly divided on the elimination of lifetime limits on medical benefits.

Even majorities of those who would repeal the law want to maintain some of those provisions. Fifty-eight percent of repeal backers favor keeping the prohibition on denying coverage based on pre-existing conditions, and 58 percent also want to continue to allow those up to age 26 to remain on their parents' policies. A substantial 40 percent minority of repeal advocates would keep the law's ban on lifetime caps on insurance benefits.

Those provisions are more popular with the country as a whole. Sixty-five percent of Americans support the ban on denying coverage based on pre-existing conditions, 73 percent want to let children stay on policies up to age 26, and 53 percent favor the elimination of lifetime caps.

Unpopular mandate

A 51 percent to 47 percent majority of the country opposes the requirement that all Americans carry health insurance. Political independents oppose the mandate by 55 percent to 42 percent.

Though Obama has argued that the law gives Americans security that they'll never go without coverage, and Republicans have warned that it will undermine the medical system, the poll doesn't detect much movement in public anxiety over health care.

Americans' outlook on their own health-care costs has improved modestly, though a majority remains pessimistic. Fifty- two percent say they expect medical costs to be worse in 12 months, down from 61 percent in December.

Two-thirds say they have seen no change in the quality of their care compared with a year ago, while 13 percent say they are better off and 19 percent worse off.

Americans are about evenly divided on whether they face a greater or lesser risk of losing access to insurance than a year ago, with 38 percent saying they are more worried about the possibility and 41 percent less worried. The remainder say the risk was about the same or were unsure.

The Bloomberg Poll, conducted by Selzer & Co. of Des Moines, Iowa, has a margin of error of plus or minus 3.1 percentage points.

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