Washington (CNN) Former FBI Director James Comey, a frequent critic of President Donald Trump, said in a blistering op-ed for The New York Times that Trump's efforts to "exploit the radioactive energy of racism" are "more dangerous" than he realizes.

"America has long had a radioactive racist soup in the center of our national life. Donald Trump thinks he is stirring it for political benefit. He's actually doing something more dangerous," Comey wrote in the piece published Sunday.

Comey's comments come after a series of racist tweets from Trump last month aimed at lawmakers of color and in the immediate aftermath of a mass shooting in El Paso, Texas, where a white supremacist killed at least 20 people in a shopping center -- one of two massacres by gunmen in the US in less than 24 hours. Earlier Sunday, Trump tweeted, "God bless the people of El Paso Texas. God bless the people of Dayton, Ohio," and, separately, ordered American flags to be flown at half-staff as "a mark of solemn respect" for the victims of the tragedies.

"Every American president, knowing what lies deep within our country, bears a unique responsibility to say loudly and consistently that white supremacy is illegitimate, that encouraging a politics of racial resentment can spawn violence, and that violence aimed at people by virtue of their skin color is terrorism," Comey wrote. "Mr. President, because of what you have done, you owe us more than condolences sent via Twitter. You must stop trying to unleash and exploit the radioactive energy of racism."