This is a major problem for Romney as he looks to lock down the GOP presidential nomination. From Jon Cohen at The Washington Post:

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.

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.

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.