Half of all Americans now express unfavorable views of Mitt Romney, a new high for the GOP presidential hopeful in Washington Post-ABC News polling. The deteriorating public impressions of the former Massachusetts governor foreshadow a significant obstacle for him as he tries to shift the focus of his campaign toward a potential match-up against President Obama.

Romney’s negative numbers have jumped around this election cycle, but the overall pattern is similar to his trajectory four years ago: As he became better known, his unfavorables shot up far more rapidly than his positive numbers. Negative impressions are up eight percentage points in the past week, nudging past the previous high, which occurred about the time Romney suffered a big loss in the South Carolina primary.

In the new poll, 50 percent of all adults and 52 percent of registered voters express unfavorable opinions of Romney, both higher — although marginally — than Obama has received in Post-ABC polling as far back as late 2006.

However, the biggest difference between Romney and Obama is on the other side of the ledger: 53 percent of Americans hold favorable views of the president; for Romney, that number slides to 34 percent. Positive ratings of Obama steadily improved over the course of his fight for the 2008 Democratic presidential nomination — something not evident in Romney’s ratings this time around, or last.

Obama’s relative low point among independents in 2008 was in mid-April, when 57 percent said they had favorable impressions and 37 percent had unfavorable ones. In the new poll, Romney is underwater with independents: 35 percent view him favorably, 52 percent unfavorably.

Basic popularity ratings present a challenge to the Republican front-runner as his campain attempts to focus on possible general election fight. (The Washington Post/Washington Post - ABC News poll)

Independents are more evenly divided on the president — 50 percent favorable, 46 percent unfavorable, about where they’ve been in recent polls.

Obama and Romney are also more closely matched in hypothetical head-to-head contests among all voters than they are on favorability, an indication that people use more than basic popularity to make their voting choices. In a Post-ABC News poll two weeks ago, 49 percent of voters said they would back Romney in such a contest; 47 percent said they would support Obama. But general impressions matter, and it’s often difficult to turn around negative opinions, a hurdle that just got higher for Romney.

The poll was conducted March 21 to 25 among a sample of 1,016 randomly selected adults. For the full survey, the margin of sampling error is plus or minus 3.5 percentage points.

Polling manager Peyton M. Craighill and polling analyst Scott Clement contributed to this report.