Rosie O'Donnell didn't waste any time starting off the New Year with her favorite activity -- going after Donald Trump on Twitter.

The 54-year-old tweeted Sunday that the President-elect is "mentally unstable" and that America has "less than 3 weeks to stop him."

DONALD TRUMP IS MENTALLY UNSTABLE - https://t.co/6AvhoPq1du



LESS THAN 3 WEEKS TO STOP HIM AMERICA — ROSIE (@Rosie) January 1, 2017

This is not the first time (nor the second or third time) that the comedian has attacked Trump. She has previously called him, among other things, "the worst representation of what the United States is or could be."

How did Arnold do on 'The Apprentice'?

Trump and O'Donnell have sparred publicly for over a decade, since way back in 2006 when O'Donnell publicly criticized Trump for refusing to fire disgraced beauty queen Tara Conner. But their feud boiled over during Trump's presidential campaign and took center stage during a GOP debate on Fox News.

Responding to moderator Megyn Kelly's question about Trump using derogatory language to insult women, Trump shot back: "Only Rosie O'Donnell."

O'Donnell even vowed to leave the United States should he be elected. Trump responded by saying, "We'll get rid of Rosie. Oh, I love it...Now I have to get elected because I’m going to do a great service to our country. Now it’s much more important. In fact, I’ll immediately get off this call and start campaigning right now."