Last week was a pretty good one for Mitt Romney. He moved ahead of Rick Santorum in the polls in Wisconsin. His lead in national polls of Republicans increased as well. And he continued to get prominent endorsements from star conservatives like Marco Rubio and Paul Ryan, indicating a party moving in his direction. But despite all this encouraging news, there was a cloud on Romney’s horizon: his terrible approval ratings.

How terrible? According to a new ABC News-Washington Post survey, only 34 percent of Americans view Romney favorably (“the lowest for any leading presidential candidate in ABC/Post polls in primary seasons since 1984”). And 50 percent of Americans rate Romney unfavorably—that’s higher than any unfavorability score Obama has received ever.

Needless to say, this is not what you want to hear when you’re still busy courting your party’s most extremist voters in an effort to nail down the nomination. In fact, Romney’s record-low ratings probably have a lot to do with this year’s extended primary. John McCain, for example, was doing much better at this point in 2008, with a 54-40 approval/disapproval rating—but he had already won the nomination. According to ABC/Post, Romney’s unfavorability score has been exceeded “by only one top candidate in 28 years, Hillary Clinton in 2008.” It’s no accident that the past candidate who actually exceeds Romney’s unfavorable is Clinton, who was also still in a highly competitive nomination contest at the end of March.

A big part of Romney’s problem, as ABC’s Gary Langer explains in his analysis of the new poll, is that “core Republican voters” don’t much like him—in sharp contrast to Obama, whose troubles with the Democratic base seem to be over:

Romney’s seen favorably by 62 percent of Republicans and 47 percent of conservatives overall, including 54 percent of strong conservatives. Obama fares much better on the other side of the political spectrum – 86 percent favorable among Democrats, 75 percent among liberals.

The president peaks at 91 percent favorability among liberal Democrats, vs. Romney’s 66 percent among conservative Republicans. These are like-sized groups: Liberal Democrats account for 16 percent of all adults in this poll, conservative Republicans for 18 percent.

Intensity of sentiment is an even sharper point of differentiation. Sixty-one percent of Democrats and half of liberals see Obama “strongly” favorably, the most in nearly a year. Strongly favorable ratings of Romney dive to 15 and 13 percent among Republicans and conservatives.

But Romney’s unfavorability among core Republicans probably won’t cost him that much. There’s plenty of reason to figure Mitt’s most conservative detractors will ultimately support him in the general election, especially against an opponent they dislike far more than Democrats disliked John McCain in 2008.A Public Policy Polling survey a bit earlier in March that showed Romney with an even more dismal approval-disapproval ratio (33-58) also indicated his most avid conservative detractors would back him strongly in the general election. According to PPP, Romney’s approval ratio among “very conservative” voters was 43/48. Yet the same voters preferred him to Obama 76/16. Meanwhile, Obama’s 81/11 approval ratio among “very liberal” voters is almost identical to his support from them against Romney (81/15). In other words, Romney has a hidden cushion of support.