To defeat Barack Obama in November, Mitt Romney would have to make history by overcoming a larger favorability deficit than any other modern presidential candidate.

Since 1976, no serious contender, Democrat or Republican, has watched his favorable ratings fall as low as Romney’s have in recent months. Or watched his unfavorable ratings climb as high. Or watched his overall numbers stay underwater—that is, more unfavorable than favorable—for so long.

At this rate, Romney is shaping up to be the most unpopular presidential nominee on record.

It’s no secret that this year’s Republican primary contest has hurt the former Massachusetts governor. What was supposed to be a rather orderly march to the nomination has steadily morphed into a degenerative, Sisyphean slog, thanks to the vagaries of delegate math and the GOP’s curious lack of enthusiasm for Mormon moderates from Massachusetts. George Will, for one, has grown so despondent that he is already advising conservatives to give up on the presidency and focus on Congress instead.

On the record, at least, Republican officials disagree. Their counterargument is that Romney’s conundrum isn’t all that unusual, historically speaking: both Ronald Reagan and Bill Clinton were losing to incumbent presidents right about now, and look what happened to them. And indeed, these people have a point: Clinton was trailing George H.W. Bush 25 percent to 44 percent in March 1992, according to the Gallup archives, and Reagan lagged behind Jimmy Carter by an even larger margin in March 1980: 33 percent to 58 percent. Considered in the proper context, Republicans say, Romney’s current position vis-a-vis Obama—he’s averaging about 44 percent support to the president’s 48—is actually rather robust.

There’s only one problem with this analysis: head-to-head surveys from March or April of an election year aren’t particularly revealing. The past four decades of mano-a-mano primary-season polling may suggest that Romney isn’t necessarily going to lose in November. But they don't telll us anything about whether or not he’s in a good position to win. If they did, Reagan would have succumbed to Carter; Dukakis would have beaten Bush; Bush would have defeated Clinton; and Kerry would have dethroned George W. In each of these elections, the eventual loser was leading the eventual winner as winter turned to spring.

So to get a better sense of how strong (or weak) Romney will be, forget about the head-to-head numbers for a few months; right now, they say more about Obama’s vulnerability than Mitt’s might. The more suggestive metric—the one that actually has some history of predicting a challenger’s general-election prowess—is his favorable rating.

Unfortunately, this is terrible news for Mitt, because he currently boasts the worst primary-season favorable-unfavorable split of any major-party nominee of the last 36 years (at least). There have been roughly 20 polls released in the last two months; only one gives him a positive favorable rating. The rest of the surveys show Romney’s unfavorables outstripping his favorables, often by as many as 20 percentage points. On five occasions, his unfavorable rating has topped 50 percent; his favorable rating has fallen into the 20s five times as well. As of March 12, when the last of these polls was released, Romney was averaging 49.6 percent unfavorable to 37.6 percent favorable—a gap of 11.7 points.

The depth and duration of Romney’s favorability dip is unprecedented, even during a heated primary battle. More often than not, the eventual winner enjoys positive favorable ratings the March-April before the election. Carter was at 74 percent favorable on March 27, 1976; Reagan was at 41 percent favorable to 34 percent unfavorable on April 18, 1980; George W. Bush was at 63 percent favorable to 32 percent unfavorable on March 10, 2000 (a split that was unchanged a month-and-a-half later); and over the month of March 2008, Obama’s favorable rating outstripped his unfavorable rating by an average of 19 percentage points.

Even some eventual losers have been known to post positive favorable ratings at this point in the contest. As of April 23, 1976, more than 60 percent of voters had a favorable impression of Gerald Ford, the incumbent president; his unfavorables were way down in the mid-30s. Michael Dukakis boasted a 38-14 favorable/unfavorable split on May 17, 1988, at a roughly equivalent stage of that year’s nomination fight. And Al Gore, John Kerry, and John McCain all enjoyed very healthy favorable ratings during primary season—ranging from +14 for Gore on April 30, 2000 to +40 for McCain on March 16, 2008—before going on to lose on Election Day.

But while a net-positive favorable rating in March or April doesn’t always guarantee victory in November, having a net-negative favorable rating, like Romney’s, has almost always led to defeat. Consider the record. In mid-April 1980, Carter’s numbers were stuck at 43 percent favorable to 48 percent unfavorable; he wound up losing to Reagan by 440 electoral votes. By the end of March 1984, Walter Mondale had slipped from 40 percent favorable and 31 percent unfavorable—his February stats—to 32 percent favorable and 37 percent unfavorable; the only state he won in November was Minnesota. On February 27, 1996, Bob Dole posted an anemic 23 percent favorable rating, and his numbers had not improved much by May, when they registered at 27 percent favorable to 29 percent unfavorable. Even though Dole eventually dug out of his hole, he still lost the general election by more than 8 million votes. History shows that if voters don’t like you in the spring, they tend not to vote for you in the fall.

As far as I can tell—I sifted through 36 years of newspaper clippings and Gallup polling archives for this story—only one candidate with net-negative favorable ratings in March and/or April has gone on to win on Election Day: Bill Clinton. In 1992, Clinton’s unfavorables topped his favorables from the beginning of March to the end of April, according to Gallup, before slipping underwater again between May 20 and July 17 (with a brief, shallow reversal around July 1). Then, after the Democratic National Convention in New York, Clinton’s numbers jumped to 62 percent favorable, 25 percent unfavorable and stayed strongly positive for the rest of the race.

So Romney just has to follow in Clinton’s footsteps and voila, the White House is his, right? Not quite. The trouble for Romney is that his favorability deficit is significantly larger than Clinton’s ever was. Right now, Mitt’s unfavorables average 49.6 percent; Bill’s never once rose that high, at least in the Gallup poll. Mitt’s favorables, meanwhile, currently average 37.6 percent; after April 12, 1992, Bill’s never dipped below 40. Clinton never posted any unfavorable numbers in the 50s or favorable numbers in the 20s; again, Romney has done both, five times apiece. In other words, more people liked Clinton back then than like Romney now, and fewer people disliked him, which suggests that Clinton’s post-primary popularity reversal was easier than Romney’s is likely to be. Clinton was a fresh, unfamiliar figure who turned out to be the most persuasive politician of his generation, and even then, he captured only 43 percent of the vote on Election Day 1992. Romney, meanwhile, has been running for president since 2006; voters have had a lot more time to make up their minds about him. Unless Mitt suddenly becomes as charming as Clinton—or suddenly convinces Ross Perot to enter the race—the task ahead of him may be the most daunting any modern nominee has ever faced.

Does this mean Romney is doomed? Hardly. His favorable ratings will rise. Right now, Newt Gingrich and Rick Santorum’s supporters are telling pollsters they dislike Romney. But after Mitt clinches the nomination and the party unites around him, many of his former foes will miraculously see the light. That said, it’s worth remembering that Romney’s favorables don’t just reflect Republican sentiment; they also reflect his first impression on independents, who awarded Romney a dismal 27 percent favorable, 49 percent unfavorable rating in the latest Pew poll. Intraparty factionalism is relatively easy to cure; a convention usually does the trick. Once set, however, swing-voter dislike can be more difficult to dislodge.

To win in November, Romney will have to make history. Not by being one of the wealthiest men elected president, or even the first Mormon, but by changing more minds that are more deeply set against him than any other nominee in recent memory. If he can’t convince the unprecedented number of voters who have already decided they don’t like him that, actually, they do, he will be heading back to Massachusetts—or New Hampshire, or California, or Utah—come November 6.