BOSTON - U.S. Sen. Elizabeth Warren, D-Massachusetts, said President Donald Trump's decision to ask Attorney General Jeff Sessions to resign makes it more important that Congress pass legislation protecting special prosecutor Robert Mueller.

"I think he's trying to foam the runway for getting rid of the special prosecutor," Warren said, speaking to reporters in Boston on Thursday. "It seems pretty obvious to me."

Trump on Wednesday asked Sessions to resign and replaced him with Matthew Whitaker as acting attorney general, until Trump can nominate a permanent replacement.

Whitaker has been critical of Mueller's investigation into Russian interference in the 2016 election, saying the scope of the investigation should be limited. Whitaker has suggested previously that the attorney general could cut Mueller's budget to the point that his investigation is halted.

Warren, a leader in the anti-Trump movement and the progressive wing of the Democratic Party, is considering a 2020 presidential run.

Warren suggested that Session's firing was a precursor to eliminating Mueller's authority. "Jeff Sessions has been with Donald Trump on every single issue except the fact that Sessions has protected the special prosecutor at a time when Donald Trump is under investigation, and he obviously doesn't like that," Warren said.

Warren said it is now up to the Senate and the House to "immediately pass legislation by overwhelming majorities to protect the special prosecutor." "This should not be a partisan issue," Warren said.

"There is an ongoing investigation, and the president of the United States should not be in a position to influence it or to be able to shut it down," Warren said. "This is a rule of law question. No one is above the law, not even the president of the United States."

In April, the Senate Judiciary Committee approved a bill, with Democratic and Republican support, that would prevent Trump from firing Mueller without cause and allow for judicial review if Mueller is fired. Republican leaders in the House and Senate did not bring the bill up for a vote.

On Tuesday, Democrats took control of the House, and Republicans retained control of the Senate.

Warren said the dismissal of Sessions shows that Mueller's investigation is at risk.

"We have a responsibility, the House and the Senate, to protect the special prosecutor from the president of the United States," Warren said. "The person being investigated doesn't get to shut down an investigation of himself. That's not how it works in the U.S."