DETROIT — In a low-key barbecue joint downtown, Hillary Clinton’s closest supporters watched her get smoked in the Michigan primary.

Mouths agape, staring upward at the glow of seven televisions that ringed the bar, these Michigan Democrats were transfixed for three hours by results that came in. They were showing the unthinkable: a persistent lead by Bernie Sanders that not only didn’t diminish, but continued to expand.

Michigan was supposed to be an easy win for Clinton. Polling in the days up to the election showed Clinton up by nearly 30 percentage points. Instead, the Democratic voters of the state served up an enormous upset win.

It seemed to surprise even Bernie Sanders himself, who held a hastily arranged press conference in his Miami hotel.

“I want to take this opportunity to thank the people of Michigan, who... repudiated the polls that had us 20, 25 points down a few days ago; who repudiated the pundits who said that Bernie Sanders was not going anywhere.”

In Detroit, Clinton’s door-knockers and phone bankers gathered along with members of Michigan’s Democratic elite. Former Sen. Carl Levin, who represented the state for decades and became the powerful chairman of the Senate Armed Services Committee, made an appearance. Clinton herself had already moved on, to Ohio, where voting will take place next week.

All eyes were on the televisions above the bar, which alternated, left to right, between MSNBC and CNN. At first, as early results came in, there was general cheer. Then Hillary Clinton’s speech from Cleveland was bumped from live coverage, thanks to the networks’ insistence on carrying Donald Trump’s long press conference in real time, and in full.

And even when one of the networks began to air Clinton’s remarks, delayed, the volume on the televisions was set so low than supporters had to struggle to hear what she was saying.

The Clinton campaign had evidently expected a larger turnout at the R.U.B. BBQ Pub in downtown Detroit. But the sparsely attended event foreshadowed a poor showing in the state that evening: a second floor reserved exclusively for the event was totally empty.

“It’s too early to say,” muttered one Hillary supporter, to me, with 48 percent of the return in, and his candidate down by more than 23,000 votes. It would only get worse as the night went on. “Wow,” was all another supporter could manage.

As colorful beer bottles lined the walls, below blue Hillary signs, supporters munched on a cookout spread: There was a barbecue chicken buffet that most dined on, but as the evening developed, the mood turned to even more devious comfort foods—people started ordering double chocolate cakes.

“The more numbers that come in, the more [Bernie] goes up!” bemoaned a Clinton-backer, in disbelief. At 11 p.m., hope for a late comeback dimmed even further. Television stations that had sent down satellite trucks for live standups began to leave.

“I’m still in shock,” said Patricia Harris, a Detroit volunteer, as NBC and the Associated Press projected Sanders as the winner of Michigan. When I pointed out that the race had been called, she refused to believe it. “I’m going to need to look at that closer,” she said.

Ultimately, 99 percent reporting, Sanders led by just under 20,000 votes, or 1.7 percentage points, beating Clinton 49.9 percent to 48.2 percent.

But by the time the results were announced, the bar was nearly empty except for the diehards and the delusional: No one wanted to be at a victory party for the loser.