Washington(CNN) President Donald Trump on Tuesday embraced a line of defense offered by his attorney, Rudy Giuliani, that "collusion is not a crime," even though he continued to deny any alleged ties between his campaign and Moscow during the 2016 election.

"Collusion is not a crime, but that doesn't matter because there was No Collusion (except by Crooked Hillary and the Democrats)!" Trump tweeted.

For months, Trump has insisted that there was "no collusion," but his Tuesday morning tweet hints at a potential expansion of his defense amid the ongoing special counsel investigation. Speaking to CNN's "New Day" on Monday, Giuliani said he wasn't sure if collusion was a crime.

The President has repeatedly denied that there was any collusion between his campaign and Moscow, and he told The New York Times in December that "collusion is not a crime."

Legal experts, however, have repeatedly said that anyone found collaborating with Russia on the 2016 election could be charged with other crimes, such as conspiracy, and special counsel Robert Mueller's investigation has not concluded.

At a CNN town hall in April, former FBI Director James Comey, whom Trump fired, said collusion "is not actually a thing that exists under the federal laws of the United States."

Instead, Comey continued, the question is whether any Americans conspired with a foreign government to commit crimes against the US, which is a crime.

And speaking to CNN in May, Carrie Cordero, a CNN legal analyst and an adjunct senior fellow at the Center for a New American Security, said collusion isn't a crime "in the literal sense" but that there could be related criminal violations in colluding, such as receiving foreign money in a political campaign or assisting with or being an accessory to computer hacking.

Trump's tweet came hours before the start of a trial brought on by the special counsel against Paul Manafort, Trump's former campaign chairman, who is charged with 18 violations of tax and banking laws related to his foreign lobbying work. The charges do not concern Manafort's work for Trump during the campaign.

Over the weekend, Trump and Giuliani launched some of their most personal attacks to date against Mueller, claiming the special counsel personally has conflicts of interest -- including an unnamed "contentious business relationship" -- that should preclude him from running the investigation. Trump also accused Mueller of bringing on too many Democratic investigators, and the President made a false claim about the origin of the Russia investigation.

On Monday, Giuliani called on Mueller to "be a man" and disclose the alleged conflict of interest, though the former New York City mayor said he wasn't aware of what it was.