(CNN) A Drexel University professor who received death threats after posting several controversial tweets said Thursday he is resigning because the year-long harassment has made his situation "unsustainable."

George Ciccariello-Maher, a professor of politics and global studies, had been teaching his class remotely via video conference after being put on administrative leave, a move that Drexel said was for his own safety.

The professor said his resignation is effective December 31.

"This is not a decision I take lightly; however, after nearly 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," Ciccariello-Maher wrote on his Facebook account. "Staying at Drexel in the eye of this storm has become detrimental to my own writing, speaking, and organizing."

The professor said the threats began last December after he posted on Twitter: "All I Want for Christmas is White Genocide." He said the tweet was a joke, a "satirical jab at a certain paranoid racist fantasy and that white genocide does not exist."

Ciccariello-Maher drew more outrage in March after tweeting that he was "trying not to vomit" watching someone give up their first-class seat for a uniformed soldier. He said he was upset about airstrikes in Iraq that killed civilians and that his post was misrepresented by "right-wing media."

In October, the professor tweeted that "Trumpism" and the "narrative of white victimization" were to blame for the mass shooting in Las Vegas that left 58 people dead and more than 500 wounded.

Drexel said in a statement Thursday that Ciccariello-Maher was leaving to "pursue other opportunities" and that it wished him well.

"Drexel University has accepted his resignation and recognizes the significant scholarly contributions that Professor Ciccariello-Maher has made to the field of political thought and his service to the Drexel University community as an outstanding classroom teacher," the school said. "Drexel University wishes Professor Ciccariello-Maher well in his future pursuits."

The situation involving Ciccariello-Maher prompted questions about free speech on college campuses and whether the First Amendment gives faculty the freedom to speak about public issues.

The American Association of University Professors said more than 100 incidents of targeted harassment against professors have been reported on college campuses in the past year.