Last night, Mitt Romney won Ohio — a state that the GOP needs in order to win in November — by a single point. Against Rick Santorum.

Romney, by racking up a decisive win among delegates, confirmed that the nomination is within his grasp. But Santorum’s strong showing in Ohio, and his wins in other states, again revealed that key GOP constituencies aren’t willing to come to terms with Romney as their nominee, and that Romney has weaknesses that are only getting exacerbated by the battering he’s taking in the primary, ones that could haunt him in the general election.

The exit polls tell a familiar story: In Ohio, Romney won among college graduates — but lost among voters without college degrees. He won among those with incomes over $100,000, but lost among voters below that. Fewer than a quarter of Ohio voters think Romney “best understands the problems of people like you.”

More broadly, Dan Balz’s overview nicely sums up the damage Romney has sustained:

Romney is in worse shape at this point in the campaign than virtually all recent previous nominees. Demographically, his image among independent voters, the most critical swing group, is more negative now than it was when the primary battle began. He could be hurt among women. He is in trouble with Latinos, a growing part of the electorate that is tilting even more Democratic than it was four years ago. He is not as strong as he needs to be among working-class white voters, among whom President Obama has been consistently weak.

Romney will be able to reintroduce himself to key swing constituencies once he wins the nomination, so his current weaknesses could prove ephemeral. But Dems are betting that his negatives will be very difficult to shake, particularly when the Obama machine gears up to full strength and attempts to tattoo Romney permanently with the positions he’s adopted to muddle through the primary. Pretty soon, Romney will be facing an operation somewhat more formidable than Santorum’s shoestring campaign.

* Santorum’s coalition reveals Romney’s problems: Ross Douthat notes that Santorum is the first not-Romney to successfullly build a national coalition:

Santorum’s coalition is roughly the same one that Mike Huckabee tried to assemble in 2008. With a demographic mix of evangelicals and blue-collar Republicans, and a message that’s conservative on social issues but more populist than the party’s Wall Street wing on economics, it’s proven capable of delivering states from Minnesota to Mississippi, the Rockies to the Rust Belt.

This is, of course, another way of describing the coalition that can’t come to terms with Romney as the nominee.

* Romney struggles to broaden coalition: Ron Brownstein sums it up:

Tuesday’s results dramatized the inability of either candidate to consistently expand beyond the beachheads of support they have already established. In the most competitive states, Romney continued to struggle among the key elements of the party’s populist wing, particularly evangelical Christians, strong tea party supporters, working-class voters and voters who consider themselves very conservative.

* Wealthy voters put Romney over the top: Relatedly, Chris Cillizza makes a persuasive case that Romney might have lost Ohio if it hadn’t been for a significant rise in turnout among wealthy voters.

* Will conservatives pressure Gingrich to drop out? If Santorum’s team has its way, it will be able to use his strong showing yesterday to persuade leading conservatives to try pushing Gingrich out of the race, so Santorum has a chance to air out a one-on-one contest against Romney.

The question is whether Gingrich — whose presidential bid has never been all that firmly rooted in a hard-headed assessment of his chances — will listen.

* How long until Republicans accept Romney as the nominee? As Karen Tumulty notes, the delegate math is clear: Romney’s rivals won’t be able to prevent him from winning the nomination. The question is how long it will take them to acknowledge it, at what cost to Romney, and what the unwillingness to accept him tells us about his general election viability.

* Can Romney’s foes deny him victory? The counterargument: Santorum and Newt Gingrich can’t win, but they may be able to deny Romney a clear path to claiming the nomination by using their delgatest to kick the contest to Tampa.

* Conservatives worried about general election: John Fund offers the conservative case for being very, very worried about Romney’s showing last night, noting that he squeaked out an Ohio win only after carpetbombing Santorum relentlesly with ads, thanks to a massive financial advantage.

Conclusion: “if his campaign doesn’t realize that tonight’s results are real danger signals for their man — with regard to both later primaries and the November election — they are fooling themselves.”

* Obama reelect reality check of the day: Despite Romney’s struggles, this from Ezra Klein is worth keeping in mind:

Though Romney has the worst poll numbers of any presidential nominee in recent history, Obama has the worst poll numbers of any incumbent president running for reelection in recent history. And we remain a closely divided country with a very fragile economy.

I’d add that for all his silver-foot-in-mouth weakness, Romney comes across as extremely competent — Dems have not undermined impressions of Romney’s competence at all — which may loom larger in the general election than anything else.

* And is the Senate GOP backing down in contraception fight? Sahil Kapur talks to Senate Republicans and findsthat they’re somewhat less than enthusiastic about continuing the birth control — sorry, religious liberty — battles.

The question is whether conservatives will allow all this to fade away quietly.

What else?