Hillary Clinton can breathe a big sigh of relief after winning Ohio tonight.

After losing Michigan last week, Clinton suddenly looked like a much more vulnerable candidate than most observers of the race had suspected. Since Bernie Sanders had somehow overcome a double-digit polling deficit in Michigan, the thinking went, what would stop him from pulling off similar upsets across the electoral map?

Clinton's win in Ohio — called around 8:40 pm by NBC News — suggests that Sanders's come-from-behind victory in Michigan may prove something of an outlier. Winning Ohio, which many thought would be even more favorable to Sanders than Michigan, makes Clinton look like a much more secure frontrunner than she did this morning.

Sanders probably needs places like Ohio to have a shot at the nomination

Clinton went into Super Tuesday II ahead by around 200 delegates, meaning that Sanders needs to win the remaining states by an 8 percent margin to take the nomination, according to Vox's Andrew Prokop.

That means Sanders can't afford to just draw even with Clinton in these contests. He needs to win states, and win some cleanly, if he's going to catch her formidable delegate lead.

It's hard to imagine how he does that after losing Ohio. The state has demographics similar to Michigan, which Sanders won, and has a less diverse electorate than several of the major contests (California, New York) that lie ahead.

Given Clinton's Southern stronghold, it's hard to imagine a path to the nomination for Sanders that does not involve clean wins in the Midwest.

"I think we'll learn whether [Michigan] was a fluke or not," Richard Berg-Andersson of the Green Papers, which tracks delegate math, said in an interview before today's returns came in. "Ohio and Illinois have similar demographics [to Michigan]."

In late February, FiveThirtyEight's Nate Silver ran projections to gauge which states would "lean" toward each candidate if Sanders and Clinton were running evenly nationally.

Silver found that in an otherwise even contest, Sanders would be winning Ohio by 1 point. His loss there tonight, in other words, suggests he's not really matching Clinton.

Winning Ohio will also help Clinton's general election argument

For months, both Clinton and Sanders have argued that they are the candidate most likely to win a general election.

Clinton has cited her long history in the national spotlight as evidence that she can withstand Republicans' withering criticisms. Her supporters have also said that Clinton will have a broader appeal than Sanders, pointing to her wins in Nevada and Virginia.

Sanders and his supporters, meanwhile, have advanced their own arguments for why he's the more electable candidate. Before today, his best example was his win in Michigan.

This is one of the reasons Ohio is so critical: it's consistently among the most important battleground states in any general election. Clinton will be eager to cite her win in Ohio as evidence that she's best poised to win a general election.