Two scholars coined the term millennial and developed a fan base for their grim theories. Now, the surviving one sees a generational realignment happening in American politics that does not bode well for Republicans.

They called it the Crisis of 2020 — an unspecified calamity that “could rival the gravest trials our ancestors have known” and serve as “the next great hinge of history.” It could be an environmental catastrophe, they wrote, a nuclear threat or “some catastrophic failure in the world economy.”

That was in 1991.

The scholars responsible were William Strauss and Neil Howe, whose book “Generations” introduced a provocative theory that American history unfolds in boom-to-bust cycles of roughly 80 years. Their conclusions about the way each generation develops its own characteristics and leadership qualities influenced a wide range of political leaders, from liberals like Bill Clinton and Al Gore to pro-Trump conservatives like Newt Gingrich and Stephen K. Bannon.

Seems as if they were on to something. So now what?

Mr. Strauss died in 2007, before anyone could know how eerily correct “The Crisis of 2020” would be. But Mr. Howe, who now hosts a podcast and analyzes demographic trends for an investment advisory firm, is still very much in the insight business. And what he sees on the other end of the coronavirus pandemic — a generational realignment in American politics hastened by the failure of the baby boomer generation to lead the nation out of its quagmire — does not bode well for President Trump or the Republicans.

For most of the past 75 years, the Republican attitude about government has been rooted in a deep skepticism of authority that says, in essence: Success doesn’t take a village; it takes a determined individual whose government isn’t standing in the way. But that belief, Mr. Howe said, “is uniquely ill-suited to the current crisis.”

Nearly 30 years ago, when he first predicted an event like the coronavirus, Mr. Howe said the year 2020 was not a mark-your-calendar prognostication of doomsday but a round number that fit the cyclical nature of their theory: It is roughly 80 years after the last great crises of World War II and the Great Depression.

More insightful than the date itself was the assertion that historical patterns pointed toward the arrival of a generation-defining crisis that would force millennials into the fire early in their adulthood. (Mr. Strauss and Mr. Howe were the first to apply that term to those born in the early 1980s because they would come of age around the year 2000.)

More than just a novelty, their theory helps explain why some of the most prominent voices calling for political reform from left, center and right have been young — Representative Alexandria Ocasio-Cortez, 30; Pete Buttigieg, 38; Senator Josh Hawley of Missouri, 40.

And as baby boomers continue to age out of public service, the theory says, fixing the problems created by the pandemic will fall to this younger, civically oriented generation. Mr. Howe, who at 68 is a member of the cohort he is critical of, said in an interview that it was no coincidence that the boomer president and many people in his generation — especially the more conservative ones — have generally taken a more lax attitude toward the coronavirus than younger people.

Polls have found that younger Americans overwhelmingly favor a cautious approach to getting back to normal — and are more worried about the virus. This includes many young Republicans, ages 18 to 49, who were far more likely than Republicans 50 and older to say the worst of the outbreak is yet to come, according to a Pew Research Center poll last month.

“This is really the problem with Gen X and baby boomers,” Mr. Howe said. “They’ve championed this kind of individualism. They’ve championed thinking less about the community.”

On the one hand, conservatives might argue that they are the best equipped to confront a moment that feels at times as if the apocalypse is at hand. Cable news, talk radio and right-wing websites have long been full of ads for products intended to sustain people through catastrophe: investments in precious metals, home generators and supplies to can your own food.

But the peace of mind those products offer is ultimately about looking out for oneself — the kind of “me first” conservatism that developed out of America’s post-World War II boom.

Mr. Howe’s critique of today’s conservatives is shared by a growing number of younger Republicans. Rachel Bovard, the senior director of policy at the Conservative Partnership Institute, said that many in her generation wanted to see an interventionist government in areas of policy like trade and finance.

“I think that’s gone unquestioned for so long, and it’s become this national theology: Private enterprise is good. Full stop,” Ms. Bovard, 36, said. “I prize my liberty, whether it’s liberty from a tyrannical government or a tyrannical corporation.”

Mr. Howe and Mr. Strauss followed “Generations” with “The Fourth Turning,” which elaborated on looming calamity. But beyond disaster prediction, the foundation of their work is that Americans tend to develop certain traits that are fairly consistent across their generation.

In the preface to “Generations” nearly 30 years ago, they nodded to the despair that boomers sometimes felt about the character of their peers. “You may feel some disappointment,” they said, “in the Dan Quayles and Donald Trumps who have been among the first of your agemates to climb life’s pyramid.”

Mr. Howe will admit to some disappointment himself on where Mr. Trump is on life’s pyramid: “I think thus far,” he said, “it’s fair to say that Trump has not grown into the role.”

One upside to the crises at the heart of these theories is the innovation they tend to produce — an economic and social program like the New Deal, or a public health discovery like the vaccine for polio. But so far the Trump administration has been incapable or unwilling to think big about the problems at hand, critics say.

“The really bad news is we are in the grip of an administration that sees everything as marketing, spin, branding,” said David Kaiser, a former professor at the Naval War College and a historian who is a fan of the Strauss and Howe theories. “And I don’t think is really capable of thinking through a problem and acting on it.”

This skepticism that big, bold solutions will come from the Trump administration is shared even by Mr. Bannon, a fairly reliable defender of the president’s since he was pushed out of his role as White House chief strategist in August 2017. In an interview, Mr. Bannon said that the administration never took seriously the possibility that a catastrophe like the coronavirus could strike, which has led to a failure of imagination in dealing with the problem.

“You had a called shot in the beginning of this administration, and nobody paid attention to it,” he said. Mr. Bannon was a promoter of the crisis theories in “The Fourth Turning” when he was still at the White House.

“I got mocked and ridiculed by so many people. They said: ‘You can’t believe in this stuff. It makes you look like a kook,’” he said. The doubters included the president, who told Mr. Bannon that the theory was too dark for him. “He said, ‘I’m an optimist.’ I said: ‘I’m a realist. And this is reality,’” Mr. Bannon recalled.

Mr. Bannon said that instead of coming up with new programs to deal with the millions of people who may never get their old jobs back, the White House and its conservative allies were falling back on the kind of stimulus policies they purport to loathe.

Where were all the conservative businessmen who have insisted that the government get out of their way, Mr. Bannon asked? “I saw them all, once again, run to the government for bailouts,” he said.

Writing in 1997 in “The Fourth Turning,” Mr. Howe and Mr. Strauss warned that after the 2020 crisis, the party in power at the time “could find itself out of power for a generation” akin to the 1860 Democrats and 1929 Republicans.

Not everyone sees a grim ending in this crisis for Mr. Trump and the Republicans. Dick Morris, a former Clinton aide who has since become a conservative critic of the Democrats, said he believed the Strauss and Howe theory helped explain how Mr. Trump won in 2016, and how he could do so again this year.

If Mr. Trump’s victory was a rebellion of working-class voters who felt the country’s leaders had failed them, Mr. Morris said, his re-election will “hinge on who is going to rebuild the economy once this is all over, which is also Trump’s strength.”

Mr. Morris, a fan of Strauss and Howe, recalled that when he worked for Mr. Clinton during the 1992 presidential campaign, the former president told him that reading “Generations” influenced him to pick Mr. Gore as his running mate because of their closeness in age and political temperament. Three of the last four presidents are boomers — Mr. Clinton, George W. Bush and Mr. Trump, all of whom were born in 1946. The likely Democratic nominee this year, Joseph R. Biden Jr., is 77 and part of the older “Silent Generation.”

If the pandemic doesn’t break the boomer generation’s grip on American government, some see hope that it will end the brand of conservatism that has thrived during their time in power.

“Where’s my copy of ‘Atlas Shrugged?’” Mr. Bannon asked, referring to the Ayn Rand novel that conservatives often cite for its heroic portrayal of individualism and self-determination. “It’s in the shredder.”