They may not be helping themselves, but Mitt Romney’s four remaining Republican rivals appear to have wounded him. As Romney’s chances of taking the nomination have steadily climbed—up 7 points over the past seven days to 66 percent Monday—his favorability rating has plunged to a field-low negative 36.

The online conversation about Romney has been more negative than that about any of his top opponents or President Obama every day since his historic New Hampshire win last week, which made him the first Republican to open the primary season with back-to-back wins there and in the Iowa caucuses (assuming his eight-vote margin of victory in the Hawkeye State holds up in the official count expected later today). His favorability rating has been subzero ever since the New Hampshire vote, meaning negative mentions of him online outnumbered positive ones. He was the only candidate, including Obama, in negative territory on Monday.

Each day the Oracle scours 40,000 news sites, blogs, message boards, Twitter feeds, and other social-media sources to track what’s being said about candidates and to determine whether the tone of the conversation is positive or negative. Based on its findings it assigns each candidate a daily favorability rating. That rating is weighted, along with the Real Clear Politics polling average and the latest InTrade market data, to calculate each candidate’s chances of winning the Republican nomination. (See methodology here.)

In a two-candidate race, like most general elections, your opponent’s loss is your gain. So negative attacks, which tend to bring down the approval numbers of both the candidate making the attack and the candidate it’s aimed at, can pay off. If your opponent’s numbers go down more than yours, you come out ahead. But that zero-sum formula doesn’t work in a more crowded field, which may help explain why Romney’s lead has grown in the South Carolina and national polls even as his rivals’ assaults appear to have taken a toll on how his Web-goers perceive him.