Arizona Sen. Jeff Flake (R.) announced Tuesday he will not seek reelection to his seat in 2018, saying he does not believe he can in good conscience earn the Republican nomination in the current political climate.

Flake, who President Donald Trump has called "weak" and "ineffective," also said he will not run as an independent and has no plans to run for president.

"Here's the bottom line: The path that I would have to travel to get the Republican nomination is a path I'm not willing to take, and that I can't in good conscience take," Flake told the Arizona Republic in a telephone interview. "It would require me to believe in positions I don't hold on such issues as trade and immigration and it would require me to condone behavior that I cannot condone."

Flake's campaign had $3.4 million in cash on hand as of Sept. 30, according to the Republic. The Arizona lawmaker said in a Senate floor speech shortly after 3 p.m. that the "coarseness" of the national dialogue under Trump should not be allowed to become the new normal.

"We must never regard as ‘normal' the regular and casual undermining of our democratic norms and ideals," Flake said. "We must never meekly accept the daily sundering of our country—the personal attacks, the threats against principles, freedoms, and institutions, the flagrant disregard for truth or decency, the reckless provocations, most often for the pettiest and most personal reasons, reasons having nothing whatsoever to do with the fortunes of the people that we have all been elected to serve."

Flake's surprise announcement is a blow to Republican hopes to hold onto the Senate majority next year given the advantages of his incumbency. He joins Tennessee Sen. Bob Corker (R.) as another incumbent who has announced he will not seek another term next year.

Flake, who was first elected to his seat in 2012, has been one of the sharpest critics of Trump from the Republican side of the aisle, saying the president's brand of protectionism and isolationism stands at odds with conservative values.

In August, Flake expressed concerns about the direction of the GOP, as well as the "anti-immigration fervor" Trump had brought.