Former FBI Director James B. Comey on Tuesday pushed back on claims by President Trump and his allies that the bureau spied on the Trump campaign in 2016.

In an op-ed in The Washington Post, Mr. Comey assails the president as “a liar” and dismissed allegations the FBI was biased against Mr. Trump as “dumb lies” and a “conspiracy theory.”

“Millions of good people believe what a president of the United States says in normal times, that’s healthy. But not now, when the president is a liar who doesn’t care what damage he does to vital institutions,” Mr. Comey wrote. “We must call out his lies that the FBI was corrupt and committed treason, that we spied on the Trump campaign, and tried to defeat Donald Trump. We must constantly return to the stubborn facts.”

Last week, Mr. Trump suggested Mr. Comey, former FBI Deputy Director Andrew McCabe and others committed treason by initiating a counterintelligence probe into the president’s campaign during the 2016 election.

Mr. Comey wrote that when the FBI learned of Russian meddling in the 2016 election and possible ties between campaign officials and the Kremlin, it had no choice but to investigate.

“Maybe there was nothing to it or maybe Americans were actively conspiring with the Russians. To find out, the FBI would live up to its name and investigate,” he wrote, insisting the probe was done “carefully, professionally and discretely.”

Republicans have claimed the probe was initiated by Clinton loyalists within the Justice Department and aimed at stopping Mr. Trump’s campaign. They point to anti-Trump text messages exchanged by FBI employees Peter Strzok and Lisa Page, both of whom were briefly part of special counsel Robert Mueller’s probe.

Mr. Comey defended Mr. Strzok, who was fired by the bureau last year. The former FBI director said if Mr. Strzok really was out to get the president, he would have leaked the probe all over Washington, instead of keeping it a secret.

“Why wouldn’t the much-maligned FBI supervisor Peter Strzok — the alleged kingpin of the ‘treasonous’ plot to stop Trump — tell anyone? He was one of the very few people who knew what we were investigating,” he wrote.

Attorney General William P. Barr recently tapped a U.S. attorney to investigate the origins of the Russian probe. Mr. Comey said that review, led by U.S. Attorney for Connecticut John Durham, will turn up empty.

“The FBI wasn’t out to get Donald Trump. It also wasn’t out to get Hillary Clinton. It was out to do its best to investigate serious matters while walking through a vicious political minefield. But go ahead, investigate the investigators, if you must,” Mr. Comey wrote.

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