WASHINGTON (Reuters) - Americans increasingly think Democrats have a better plan for healthcare than Republicans, according to a Reuters/Ipsos poll conducted after the White House announced that more people than expected had signed up for the "Obamacare" health plan. Nearly one-third of respondents in the online survey released on Tuesday said they prefer Democrats' plan, policy or approach to healthcare, 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...