A Donald Trump supporter outside the Spokane Convention Center, where Trump was speaking at a campaign rally on May 7.  (Jake Parrish/Reuters )

With Donald Trump almost certainly the Republican Party’s candidate for president, pundits in and out of the conservative movement are predicting devastating effects for the party.

They have good reason to think so. Political scientist Stephen Skowronek’s thoughtful theory of presidential leadership suggests that, if Trump were to be elected, his presidency would end the Reagan era. Trump could be such a disastrous president that he would open the way for a transformative Democratic Party candidate to win the presidency in 2020, overcome congressional opposition, and put in place a 21st-century New Deal.

According to Skowronek’s “The Politics Presidents Make,” a president’s political success depends on where he (or, potentially, she) falls in a cycle of party dominance. The 1993 book says that five “reconstructive” presidents have come to office, overthrowing the dominant party and replacing it with their own party: Thomas Jefferson, Andrew Jackson, Abraham Lincoln, Franklin Roosevelt and Ronald Reagan. During these periods, successive presidents of the same party extend those “reconstructive” presidents’ work, while opposing presidents accommodate their winning agenda, as Bill Clinton accommodated the Reagan agenda with his “third way” approach.

Are we witnessing the end of the Reagan era, or has it already happened?

Candidate Barack Obama employed the imagery of transformation, and magazines even compared him to Roosevelt and Reagan. But we are still in the Reagan revolution. Obama quickly encountered resilient tea party opposition and provoked calls among Democrats for more fundamental change in his own party.

Reconstructive presidents generally have an easier time shattering opposition movements and setting their own party’s trajectory. Their immediate successors, like James Madison, Martin Van Buren, Harry Truman and George H.W. Bush, promised to follow in their footsteps without an insurgent challenge in their own party.

Here’s one sign that the Reagan revolution is now ending. At the end of its cycle, a dominant party can’t agree on new policies for changing times. And so its last gasp in the White House comes to symbolize outmoded politics. Skrowronek’s theory classifies late-cycle presidents as “disjunctive,” neither carrying out their standard-bearer’s policies nor successfully enacting their own. Four such one-term “disjunctive” presidents — John Quincy Adams, James Buchanan, Herbert Hoover and Jimmy Carter — have paved the way for transformational presidents. Their presidencies became known for corrupt bargains, secession, Hoovervilles and malaise. George W. Bush, with his deep roots in the party and a muscle-flexing war, did not fit this mold. But Trump would.

Disjunctive presidents come to office when the dominant party’s old solutions are no longer seen as effective, but it is not united behind new ones. Currently, Republicans can’t agree on what to do with the growing millennial and Latino populations.

Confronted with unfavorable circumstances, late-cycle parties nominate someone with tenuous links to the party to offer something different under the same party label. Adams had been part of Hamilton’s Federalist Party before acquiescing to Jeffersonian dominance. Hoover was a humanitarian “wonder” to both parties before he cast his lot with Republicans in 1920. Carter forged an independent political identity and distanced himself from both party establishments as the public recovered from Watergate.

Trump’s roots in the Republican Party are equally shallow. Until recently, Trump enjoyed close ties with Democratic politicians and enunciated liberal views on issues such as abortion, health care and foreign policy. A disjunctive president typically comes from the “region of greatest erosion in his party’s national support,” as the Northeast is for Republicans.

A Trump presidency would face a host of difficulties that could hurt the Republican Party

Trump, like his disjunctive forbears, would find himself between a rock and a hard place. Many other Republicans in 2016 ran with more legitimate claims to be Reagan’s heir. By failing to defeat Trump, they cast the relevance of Reagan’s ideological principles into question. But Trump wouldn’t be able simply to adopt liberal solutions without alienating needed allies.

Neither fish nor fowl, Trump focuses on technique, promising to take what has already been done but to do it better. He asserts that his forceful persona will reinvigorate withered policies, allowing him to claim progress without promoting or repudiating Reagan’s principles. Republicans have already plucked Reagan’s low-hanging fruit, including tax cuts and defense spending. Other goals, like cutting entitlements, are riskier and more likely to be rejected by voters. Thus, technique is what Skowronek calls the “last remaining premise for action” for late-cycle presidents. Not surprisingly, two of them (Hoover and Carter) were engineers.

As a party loses dominance, disjunctive presidents even support moderate versions of the other side’s reforms. Hoover’s Reconstruction Finance Corporation was the kind of legislation New Dealers later sponsored, and Carter heeded Republican calls for supply-side economics by deregulating the trucking and airline industries. While no one can predict what Trump will do, he recently discussed raising the minimum wage and repealing trade agreements — proposals championed by Democrats and the left — while doubling down on his proposals against immigration.

Disjunctive presidents win no friends among the other party while provoking cries of treachery from their own. As Skowronek writes, “To affirm established commitments is to stigmatize oneself as a symptom of the nation’s problems and the premier symbol of systemic political failure; to repudiate them is to become isolated from one’s most natural allies and to be rendered impotent.”

Sometimes it’s better for a party to be in the opposition than in the presidency

Outside of the White House, however, an ailing party may regroup. In the 1890s, a Democratic president’s political mistakes arguably prolonged an era of Republican dominance that stretched from 1860-1932. If Democrat Hillary Clinton wins the presidency and continues “third way” politics, Republicans may have time to adapt their party’s principles to the United States’ changing demographics.

But if Trump were to win, he would likely be a president in the disjunctive mold, with failures that would open the way for a transformative Democrat — someone more in the mold of Elizabeth Warren than Clinton. A post-Trump Democratic president would have the political capital to move full speed ahead. The president’s transformative policies wouldn’t need to work any better than did Jackson’s destruction of the Second Bank of the United States. Voters stay loyal to the new dominant party when they are happy to be rid of the discredited disjunctive president, not because the new party’s solutions are effective. After such a transformational moment, a party can remain dominant for decades, in part by reminding voters how disastrous the other party’s last incumbent had been.

When a once-dominant party elects a disjunctive president, it gains little — and hurts its  reputation, potentially for decades. Some Republicans are worried about the consequences either of supporting or opposing Trump. Those who want to keep the party’s power and reputation might choose to lose the presidency and settle for congressional opposition.

Unless the public turns away from a disastrous president, as Trump has the potential to be, a potentially transformational Democrat like Warren would find it much easier to indict Republican approaches and gather broad support for dramatic change.

Chris Baylor is a visiting assistant professor at the College of the Holy Cross and author of “First to the Party,” a forthcoming book on party transformation.

Speaker Paul Ryan has backed away from his pledge to support whoever becomes the nominee, saying he's "not ready" to endorse Republican presidential candidate Donald Trump. Other GOP heavyweights, including the Bushes, are also not giving endorsements. (Jenny Starrs/The Washington Post)