WASHINGTON – Expanding his scorched-earth defense against possible impeachment, President Donald Trump now accuses House Speaker Nancy Pelosi of "treason" and says she is the one who should be impeached.

Having made similar claims against Rep. Adam Schiff, D-Calif., chairman of the House Intelligence Committee, Trump said Pelosi has abetted unfair tactics in the investigation of his dealings with Ukraine.

"This makes Nervous Nancy every bit as guilty as Liddle’ Adam Schiff for High Crimes and Misdemeanors, and even Treason," Trump tweeted late Sunday. "I guess that means that they, along with all of those that evilly 'Colluded' with them, must all be immediately Impeached!"

Members of Congress cannot be impeached. Impeachment is a tool that Congress uses to investigate judges or executive branch officials they believe may have committed crimes.

Pelosi, Schiff and other Democrats have made clear they will continue to investigate Trump's efforts to have Ukraine and perhaps other countries investigate Joe Biden, a Democratic U.S. presidential candidate.

"It’s about a damning call in which the President pressured a foreign power to investigate a political rival, harming national security," Schiff tweeted over the weekend. "It’s about our democracy."

Some critics have accused Trump of treason – a crime punishable by death – for seeking to involve another country in the American presidential election.

Trump's attack on Pelosi came during a weekend of angry tweets against Democrats, journalists, whistleblowers and others involved in the impeachment drama.

At one point, Trump took after Sen. Mitt Romney, R-Utah, for criticizing the president over calls to have China, as well as Ukraine, investigate Biden.

Trump also called to have Romney impeached, though, again, members of Congress are not subject to impeachment.