WASHINGTON (Reuters) -- Americans increasingly believe Democrats have a better plan for health care than Republicans, according to a Reuters/Ipsos poll conducted after the White House announced that more people than expected had signed up for Obamacare.

Nearly one-third of respondents in the online survey released Tuesday said they prefer Democrats' plan, policy or approach to health care, compared to just 18 percent for Republicans. This marks both an uptick in support for Democrats and a slide for Republicans since a similar poll in February.

Secretary of Health and Human Services Kathleen Sebelius stepped down last week after overseeing the law's rollout, including the HealthCare.gov website's tumultuous first weeks, when many users were unable to access the system to purchase or research their insurance options.

But a surge of late signups for health coverage pushed the number past 7.1 million people by the end of March, and Sebelius said before resigning that more than 7.5 million were expected to sign up this year.

"In the last couple of weeks, as the exchanges hit their goals, news coverage has been more positive and the support of the Democratic Party on this issue has rebounded," said Ipsos pollster Chris Jackson.

"It's not that independents are moving their way, it's that Democrats who had previously been a little bit ambivalent in their support are coming back to the party," he said.

One-fifth of respondents said they did not know which party had a better plan, and another fifth said neither party did.

Republicans and Democrats are facing off over President Barack Obama's signature health care law ahead of November's congressional elections, when Republicans are looking to reclaim control of the U.S. Senate and bolster their advantage in the House of Representatives.

Opposition to the Affordable Care Act is a central theme in many individual campaigns, and national Republicans have promoted ending the law as the most important issue at play in the elections.

While Democrats have struggled with the law's unpopularity, Republicans have faced criticism that they do not have any reasonable alternative to Obama's health care plan.

"Democrats have not managed to have a huge lead over Republicans so much as Republicans have managed to damage their own position and stay behind Democrats," Jackson said. "That's because people don't view the Republican Party as standing for any particular health care system."

In a February poll, just around one-quarter of respondents said Democrats had a better plan. That number increased to 31 percent in March and 32 percent in April.

Republicans' health care plans had the backing of 24 percent of respondents in the March survey, 6 percentage points higher than their April support.

Ipsos polled 799 Americans online from April 6 to 15. The poll had a credibility interval - a measure of precision - of 4 percentage points.

(Editing by Alistair Bell and Eric Walsh)

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