In early March, when the number of American coronavirus cases was still low and the president himself sounded unalarmed, Republicans generally told pollsters they were unworried, too. About 40 percent of them in a national tracking poll said they were not at all concerned about an outbreak in their communities. In that same moment, hardly any Democrats agreed.

That pattern, consistent across other surveys, seemed to suggest not long ago that Americans would view even a deadly infectious disease as they have come to see so many facets of life — refracted through pure partisanship.

But more recently this picture has begun to shift.

“Not concerned at all” about a coronavirus outbreak in their local area Source: Civiqs

As the coronavirus toll has risen and the stock market has plunged, as the economy and daily life have come to a halt, and as the president and conservative commentators have changed their tone, Republican concerns about the virus are now rising. Political scientists expect that in short order the views of Republicans and Democrats may converge on how seriously they take the virus.

“While the effects of partisanship are incredibly pronounced, I think they also hit their limits,” said Brian Schaffner, a political scientist at Tufts.

In the months ahead, we may well see persistent partisan splits on whether Americans approve of how the president is handling the crisis. But on the more fundamental question of how they perceive the threat in their personal lives, Democrats and Republicans are now moving in the same direction.

Both say they are growing more concerned about an outbreak in their local areas, according to a daily tracking poll by Civiqs. Since the stock market crash on March 9, the share of Americans of both parties who said they were “extremely concerned” about an outbreak has increased nearly every day.

Partisan differences are eroding in part because the messages coming from President Trump, Fox News hosts and other Republican party leaders increasingly — if inconsistently — resemble the messages Democrats are hearing from their preferred news sources and leaders. Even some policy responses, like sending Americans money to weather the crisis, have received bipartisan support.

“At some point, if everybody is saying the same thing,” said Ryan Enos, a political scientist at Harvard, “there’s reason to believe that that party gap should go away.”

Some of these partisan differences may be because Republicans are more likely to live in rural communities, far from the first domestic outbreaks and in places where social distancing isn't such a challenge. But the cues people get from party leaders are also hugely important in shaping public opinion. And the president has gone from dismissing the virus as under control to addressing its dire consequences in daily briefings — all messages that have in turn been amplified by conservative media.

In responding to pollsters, both Democrats and Republicans have a tendency to “cheerlead” for their party — to give the answer that their team is supposed to give, even if that answer doesn’t reflect their true behavior, or if it contradicts indisputable reality. That may be part of what explains these early and stark differences in coronavirus concerns. Until quite recently, if you were a Fox News watcher, you weren’t supposed to be worried about the virus.

Mr. Schaffner, the Tufts political scientist, wondered if he could bypass what people were saying and look directly at how Democrats and Republicans were behaving. He found something like that evidence in online search data. In early March, people in Democratic-leaning media markets were much more likely to do a Google search for hand sanitizer — a trend that didn’t exist before the crisis.

Over the last two weeks, that partisan pattern all but disappeared:

↑ More searches for hand sanitizer More Repulican areas → In early March, places that were more Republican were much less likely to search for hand sanitizer. As the outbreak became more serious, and as party leaders communicated the scale of the problem, that divide diminished. Circles represent media markets; they are sized proportional to the number of households in each market. Google Trends are an index. · FiveThirtyEight, Nielsen Sources: Google Trends

There is also some evidence that Republicans in parts of the country hit first by the virus began to grow more concerned even before the president’s cues shifted. A national poll conducted by Survey 160 and Gradient Metrics, which oversampled Washington State residents, found that Republicans there were much less likely than Republicans nationally to say the crisis had been exaggerated by the media. On that question, Republicans in Washington looked more like independents across the country.

In state data from the Civiqs daily tracking poll, Republicans in Washington State have also shown for weeks the highest levels of concern.

“Extremely concerned” about a coronavirus outbreak, state by state Source: Civiqs

That suggests that people in other parts of the country will grow more worried, regardless of their partisanship, as they learn more not just from TV anchors and presidential news conferences, but also from their sick friends and neighbors.

“At a certain point, if they don’t know someone who has it, they’re going to know someone who knows someone who has it,” said Samara Klar, a professor at the University of Arizona. “And that’s not going to fall along party lines. Those kinds of things are going to make the partisan gap go away.”

What’s perhaps more remarkable is not that the gap will ultimately narrow, but that it existed so starkly in the first place. Similar ideological divides haven’t been apparent in Britain. In the U.S., differences in level of concern between Democrats and Republicans — or between people who get their news from national newspapers or Fox News — have been far wider in YouGov polls than any of the differences by education, income, age or race.

Past public health threats in America have shown some echoes of this partisan pattern. Republicans, for example, were more likely than Democrats to say they were concerned about the Ebola outbreak that occurred during the Obama administration.

But in the past, Democrats and Republicans behaved in some similar ways when faced with a public health threat, according to work by Bethany Albertson, at the University of Texas at Austin, and Shana Kushner Gadarian, at Syracuse University. They have studied the role of anxiety in politics when people confront threats like terrorism and disease. Facing a public health threat, they found, the more anxious Democrats and Republicans became, the more likely they both were to trust expert sources like the Centers for Disease Control and Prevention.

“People want expert actors; they don’t want partisan actors in times of crisis,” Ms. Gadarian said. “What’s different now than in our studies is that our current president has been undercutting messages from the C.D.C. and other health experts about, first, whether or not this is a crisis, whether you should be anxious, and, second, what we should be doing.”

She and Ms. Albertson also suggest partisanship among the public and the media had grown more pitched than during H1N1 or even Ebola.

“For weeks and weeks, this threat has been politicized, or made partisan, in a way that is quite stunning,” Ms. Albertson said.

Consider the counterfactual where that trend continued, even in the face of a rising death toll and widening restrictions on daily life.

If Trump “had made less of a dramatic shift, you have to think about scenarios where conspiracy theories would be gaining more ground, the idea that ‘my kids’ schools were closed and it’s the Democrats’ fault,’” said Matt Grossmann, who directs the Institute for Public Policy and Social Research at Michigan State. “These are all possible alternative worlds if the parties weren’t converging in concern and threat at the moment.”