Sen. Jeff Flake of Arizona, President Donald Trump’s most vocal Republican adversary in the Senate, cautioned in his farewell address Thursday that the “threats to our democracy from within and without are real.”

Flake declined to seek re-election to a second term, acknowledging that his battles with Trump over the past two years made it unlikely that he could survive a primary challenge. While Flake did not mention Trump in his speech Thursday, he said that describing the state of the nation’s politics as “not healthy is something of an understatement.”

“We of course are testing the institutions of American liberty in ways that none of us likely ever imagined we would — and in ways that we never should again,” Flake said.

Over the past two years, Flake has made it a point to call out Trump in his writings and speeches, accusing him of “reckless, outrageous and undignified behavior” and saying that his use of the terms “fake news” and “enemy of the people” was reminiscent of words infamously used by Soviet dictator Josef Stalin.

It’s unclear what role Flake could have in the Republican Party in the future. He has traveled to New Hampshire to speak to voters there and has not ruled out a run for the presidency, though he would be a huge underdog going into the race.

Flake, 55, served in the House for six terms and in the Senate for one term. He was one of the earliest and most prominent critics of spending that lawmakers diverted to their congressional districts, called earmarks. He was also an outlier among Republicans when it came to supporting an end of travel and trade restrictions with Cuba.

His push for an overhaul of the nation’s immigration laws alienated many Republican voters who viewed it as rewarding people who came to the U.S. illegally. But it’s his criticism of Trump that most angered the Republican base.

That was a point Trump was all-too-happy to make on Twitter after Flake made his decision to not seek re-election, also mentioning that outgoing Sen. Bob Corker, R-Tenn., was in a similar position.

“The reason Flake and Corker dropped out of the Senate race is very simple, they had zero chance of being elected. Now act so hurt & wounded!” the president tweeted at the time.

Flake also used his speech to warn about global events. He said Russia’s President Vladimir Putin hijacked democracy in his own country and is determined to do the same globally.

“Denial of this reality will not make it any less real,” Flake said. “This is something that is staring us in the face, right now, as we are gathered here today.”

Flake said the U.S. is a beacon to the world and that it must recognize that “the shadow of tyranny is once again enveloping parts of the globe.”

“Let us recognize as authoritarianism reasserts itself in country after country that we are by no means immune,” Flake said.

(AP)