Arizona's Flake, in New Hampshire, mulls 2020 challenge to Trump

Retiring U.S. Sen. Jeff Flake, an inveterate critic of President Donald Trump, charged in a Thursday speech to the National Press Club that his fellow Republicans “might not deserve to lead” because of their blind loyalty to the president.

The Arizonan also said he could wage a 2020 GOP primary challenge against Trump.

"It's not in my plan to run for president, but I am not ruling it out. Somebody needs to stand up for traditional Republicanism," Flake said. "Somebody needs to raise that, for nothing else than to give people hope that that decent party will be back. We'll get through this."

"It's not in my plan to run for president, but I am not ruling it out. Somebody needs to stand up for traditional Republicanism."

— U.S. Sen. Jeff Flake, R-Ariz.

Flake's comments came on the eve of his first solo political appearance in New Hampshire, the state expected to host the nation's first presidential primary election in less than two years. Flake will deliver a speech Friday morning titled "Country Over Party," as part of the esteemed "Politics and Eggs" speaker series at Saint Anselm College.

A full year before presidential candidates typically begin courting local voters, there is already an expectation among top Republicans that Trump will face a challenge from within his own party in the next presidential contest. Yet few think Trump could be defeated, even under the worst circumstances.

"It's virtually impossible to beat an incumbent for the nomination,” said Steve Duprey, who represents New Hampshire at the Republican National Committee. “But that doesn't prevent people from trying with various degrees of seriousness."

Flake acknowledged that Trump is too popular among the Republican base to lose a primary in the current political climate, but suggested that a disastrous mid-term for the GOP could realign voter loyalty.

"Things can unravel pretty fast," Flake said.

And if Trump's standing with the base doesn't fade, Flake would consider a presidential bid as an independent.

"I'm not ruling that out either," he said. "There are going to be a lot of other people in the party looking for something else."

On Monday, just three days after Flake's visit, Trump is expected to make his first appearance in New Hampshire since winning the 2016 election.

The Associated Press contributed to this report.