The threats began last December, when Drexel University professor George Ciccariello-Maher tweeted that all he wanted for Christmas was white genocide.

This week, he resigned, after a year of enduring unrelenting harassment and death threats for his controversial tweets, he said.

"After a year of harassment by right-wing, white supremacist media outlets and internet mobs, after death threats and threats of violence directed against me and my family, my situation has become unsustainable," he wrote in a statement on Facebook.

Dozens of incidents of harassment against professors have been reported on college campuses in the past year, with African-American professors among those most targeted, according to the AAUP. Conservative-leaning websites have drawn attention to professors they allege "discriminate against conservative students and advance leftist propaganda in the classroom," according to one website, "Professor Watchlist." Critics say the websites are an attack on academic freedom.

The Christmas tweet was meant to be satirical, as white genocide is an "imaginary concept" used by the far right to scare white people, Ciccariello-Maher said.

In April, the politics and global studies professor again made headlines when he criticized someone giving up their first class seat on a plane to a uniformed soldier.

He cited Mosul in reference to an airstrike in March by U.S. forces, which may be among the worst U.S.-led civilian bombings in 25 years.

And in October, he again sparked outrage with a series of tweets that suggested the Las Vegas shooting, which killed at least 59 people and injured more than 500 others, was brought on by the "narrative of white victimization."

Ciccariello-Maher on Thursday announced his resignation from Drexel, effective Sunday. He wrote in his statement on Facebook that the decision was not one he took lightly and that his position at the university has become "unsustainable."

"Staying at Drexel in the eye of this storm has become detrimental to my own writing, speaking, and organizing," he wrote.

He told CNN earlier this month that he had 800 unread voicemails in his inbox and that the threats involving his child "are the most frightening to me."

His October tweets about the Las Vegas shooting led Drexel to place Ciccariello-Maher on administrative leave for safety purposes, according to Inside Higher Ed. Drexel administrators, who have tried to distance the university from Ciccariello-Maher's positions, said they were concerned about the growing number of threats directed toward him, and Ciccariello-Maher has since been teaching his courses online.

Ciccariello-Maher explained his tweets a Washington Post op-ed, saying his argument was "not new, but rather reflects decades of research on how race and gender function in our society."

The American Association of University Professors came to Ciccariello-Maher's defense, saying the university bowed to the pressure of those threatening him.

"A suspension is a severely adverse personnel action, and imposing one on Ciccariello-Maher without consulting an appropriate faculty body raises concerns for his academic freedom and tenured status," the group said in a statement. "It is especially concerning that the suspension is indefinite.

In the past year, conflicts over free speech on college campuses have escalated as clashes have broken out between white nationalists and counterprotesters. In August, a rally by several hundred white nationalists and white supremacists at the University of Virginia resulted in shoving, punching and the spraying of chemical irritants by both groups, and a woman was killed when a car drove into a crowd of counterprotesters.

Protests of controversial speakers like Milo Yiannopoulos and Richard Spencer have turned violent, and a fifth of undergraduate students now say it's acceptable to silence a speaker with physical force if they make "offensive and hurtful statements," according to a September survey of students conducted by the Brookings Institution.

University professors have also faced increased scrutiny with classroom conversations exposed on social media and as conservatives rallied against what they called left-leaning colleges and universities.

Last December, death threats forced an Orange Coast College professor to flee her home state of California after a video surfaced of her telling her students that President Donald Trump's election was an "act of terrorism." In June, an assistant professor at the University of Iowa was hit with hate mail and threats for her piece discussing how classical white marble statues were often painted. Stories of her work began to circulate online, with one reading, "Prof: 'white marble' in artwork contributes to white supremacy."

In his Facebook post addressing his resignation, Ciccariello-Maher said tenured faculty should defend all faculty members from attacks by the far right and white supremacists.

"Only then can we build campus solidarities that transcend such artificial boundaries among faculty — and beyond, to campus workers and students as well — solidarities that will be the last line of defense in what is today a losing battle for universities," he wrote.

He also encouraged his students to keep standing up for their rights by marching and protesting.

In a statement Thursday, university officials said they accepted Ciccariello-Maher's resignation and wished him well. Officials said they recognize his "significant scholarly contributions" and service as an "outstanding classroom teacher."

"Drexel University wishes Professor Ciccariello-Maher well in his future pursuits," officials said.