MANCHESTER, N.H. — Sen. Jeff Flake (R-Ariz.) received a standing ovation from a crowd of business leaders and political junkies here on Friday after decrying the “degradation of the United States and her values” by the current occupant of the White House.

Ohio Gov. John Kasich (R), who ran for the GOP nomination in 2016, will return to the nation’s first primary state early next month for a “fireside chat” at a college in Henniker.

And on Monday, President Trump — whose New Hampshire primary win two years ago set him on a course for the presidency — is slated to make an appearance in the state for the first time since 2016. 

The next presidential primaries here are nearly two years away, but the unusual flurry of activity is stoking speculation about whether a sitting president could face a serious challenge from within his own party for the first time in a quarter-century.

In 1992 — the last time that happened — Pat Buchanan’s strong GOP primary showing here helped weaken incumbent George H.W. Bush, who went on to lose reelection against Democrat Bill Clinton.

“I’m certain that Trump will draw a serious primary challenger,” said Fergus Cullen, a former chairman of the New Hampshire Republican State Committee. “A lot of voters are getting tired of this act.”

Trump’s Monday visit is billed as an official trip, with a focus on combating the opioid crisis, which has hit the Granite State hard. But amid the ongoing Russia probe and fallout from an alleged dalliance with a porn star, the appearance also presents an opportunity for the president to shore up support in a state that not only plays a key role in the nominating process but also is a general-election battleground.

Vice President Pence also plans to plant the flag in New Hampshire later in the week, speaking at a fundraiser for Gov. Chris Sununu (R) and at a gathering of the pro-Trump group America First Policies.

Aside from scandals in Washington, Trump has faced particular challenges here, including fallout from his claim that there was “serious voter fraud” in New Hampshire in 2016 and his characterization of the state as a “drug-infested den.” Democrats — including New Hampshire’s senior senator, Jeanne Shaheen — have also accused Trump of paying lip service so far to the opioid crisis, which as a candidate he promised to address aggressively.

A poll last month showed sagging support for Trump here, with only 36 percent of voters approving of his performance in a state he narrowly lost in 2016 to Democrat Hillary Clinton. That number is more than 20 points lower than the job approval of the state’s Republican governor, according to the poll by the New Hampshire Institute of Politics at Saint Anselm College.

Sensing vulnerability, potential Democratic challengers, including former vice president Joe Biden and Sen. Bernie Sanders (I-Vt.), have started making state appearances, and the parade of hopefuls is expected to pick up considerably after the November midterm elections.

John Weaver, a Kasich adviser, cautioned against reading too much into the Ohio governor’s scheduled April 3 return to New Hampshire. But Weaver said he thinks Trump will face a GOP challenger in 2020, whether it’s Kasich or someone else. Kasich finished second to Trump in the 2016 Republican primary here.

“No one would have predicted Trump would be president,” Weaver said. “No one should predict whether Trump will be president in three months, in six months or running for reelection. The rapidly changing landscape should be observed and not predicted.”

Despite his low popularity among the general electorate, Trump’s standing remains relatively strong among likely Republican primary voters here.

As of last month, 60 percent of Republican primary voters planned to vote for Trump in 2020, according to the Granite State Poll conducted by the University of New Hampshire. That’s up from 47 percent in the same poll in October.

Even Flake, who delivered a blistering speech about Trump’s presidency here on Friday, conceded that Trump remains too popular among the GOP base to lose a primary if it were held today.

But Flake told reporters that “things can unravel pretty fast,” particularly if Republicans lose control of Congress in the midterms.

“Two years from now, we could have a completely different scenario,” Flake said Sunday during an appearance on CNN’s “State of the Union.” “Nothing focuses the mind like a big election loss.”

In his speech at the Politics & Eggs breakfast — long viewed as an obligatory stop for potential presidential candidates — Flake said he hopes a serious Republican does challenge Trump.

“I think that the Republicans want to be reminded what it means to be a traditional, decent Republican,” he said.

The standing ovation Flake received was rare, according to regular attendees of the long-running series. Christopher Galdieri, a professor of politics at Saint Anselm’s, said the only other speaker he could recall receiving a similar reaction was Rep. John Lewis (D-Ga.), the civil rights icon.

Trump loyalists say they are not fazed by the activities of potential GOP challengers or by Trump’s low standing among the general electorate here.

“New Hampshire’s a fickle state,” said Barry Bennett, who advised Trump during the 2016 general election. “They don’t fall in love with anyone. They only flirt.”

Patrick Hynes, a state GOP spokesman, called recent polling on Trump’s popularity a “lagging indicator” of his standing with New Hampshire voters. He predicted that Trump would gain more credit for the Republican tax overhaul, as well as for bringing North Korean leader Kim Jong Un to the negotiating table.

“As that message sinks in, his numbers will continue to go up,” Hynes said.

Trump has started gearing up for his reelection bid. He is actively raising money, and he recently named a campaign manager and other staffers. Such moves further raise the entry barrier for potential challengers.

Traditionally, when sitting presidents have faced primary challenges, those challenges have come from the party’s base — from the left wing of the Democratic Party or the right wing of the Republican Party. In Trump’s case, it appears a challenge would be less on ideological grounds than over his ability to govern by accepted norms.

Primary challenges to sitting presidents face long odds, but they have served to weaken the incumbent.

In the 1992 New Hampshire primary, Buchanan relentlessly attacked Bush for having broken his “read my lips” pledge not to raise taxes, and he won 38 percent of the New Hampshire primary vote. His challenge was widely seen as undermining Bush heading into the general-election campaign against Clinton.

Similarly, the Democratic challenge by then-Sen. Ted Kennedy (Mass.) in the 1980 primaries contributed to Jimmy Carter’s defeat against Republican Ronald Reagan in the fall. And Reagan’s 1976 GOP primary challenge to Gerald Ford was a factor in Ford’s loss in the general election that year.

Kathleen Sullivan, a former chairwoman of the New Hampshire Democratic Party, said Trump’s low popularity has contributed to unusually high interest among potential Democratic challengers, some of whom have already visited or reached out more quietly to prominent activists.

“I think his support is collapsing except among those people who were Trumpists before Trump even thought about running,” Sullivan said.

Last month’s Granite State Poll found Biden to be the most popular among potential Democratic candidates, drawing 35 percent of the support from primary voters. He was followed by Sanders, with 24 percent, and Sen. Elizabeth Warren (D-Mass.), with 15 percent. All other names in the poll registered in single digits.

Sanders and Biden have stopped by the state during the past year. The most frequent Democratic visitors have included a pair of Marylanders: Rep. John Delaney, the only declared candidate; and former governor Martin O’Malley, who ended his 2016 bid before the New Hampshire primary.

Others Democrats who have made appearances include Rep. Tim Ryan (Ohio); Jason Kander, the former Missouri secretary of state; Julián Castro, the former Obama Cabinet member and San Antonio mayor; Los Angeles Mayor Eric Garcetti; Rep. Seth Moulton (Mass.) and Sen. Jeff Merkley (Ore.).

“We have a country that needs leadership, and we’re crying out for it,” said state Sen. Lou D’Allesandro, a longtime Democratic power broker in New Hampshire.

In addition to a Monday speech, Trump plans to stop at a fire station in D’Allesandro’s district where opioid addicts can seek help without fear of arrest. D’Allesandro said he has no plans to greet the president.

“I have no interest at all in seeing him,” D’Allesandro said. “I have work to do.”