Hundreds of lawyers have signed onto an open letter criticizing Senate Majority Leader Mitch McConnell for his comments saying the Senate's upcoming impeachment trial does not have to be impartial.

In the letter to the Senate, published Tuesday by the group Lawyers Defending American Democracy, the lawyers said that McConnell's "assertions cannot withstand scrutiny."

"Just because the Constitution commits the impeachment process to a 'political' branch of government and senators may legitimately promote their partisan self-interest as part of the legislative process, does not mean they are permitted to do so when serving as judges and jurors in an impeachment trial," the letter reads. "To the contrary, as the adjudicators of impeachment, they have a duty to serve in a quasi-judicial capacity."

The letter was published Tuesday morning. And Tuesday afternoon, McConnell announced he had secured enough votes to start the Senate's impeachment trial without passing rules that guarantee witnesses can be called or new evidence can be produced, provisions Democrats had sought.

That move echoes the rules for President Bill Clinton's Senate impeachment trial, in which guidelines on witnesses and evidence was debated after opening arguments were held and questioning began in the trial.

A shock announcement from national security adviser John Bolton on Monday—that he would agree to testify in the Senate impeachment trial under subpoena—was initially thought to pressure some moderate Republicans in calling for McConnell to let them subpoena Bolton and other witnesses.

But McConnell's announcement Tuesday effectively brought to a close speculation about the impact Bolton's announcement would have on the Senate trial rules.

In Tuesday's letter, the attorneys took specific aim at McConnell's statement that there is "not anything judicial" about impeachment. They point to the Senate's standing rules for impeachment trials, "many of which parallel judicial proceedings."

"In short, Leader McConnell's notion that the impeachment process does not have judicial character and implicitly gives him and other senators free rein to conduct the trial as biased political partisans is indefensible," the letter reads. "To the contrary, all senators have a solemn duty to do 'impartial justice' in all aspects of the impeachment trial. This includes Leader McConnell."

The letter's signatories include a number of prominent attorneys, including former Latham & Watkins global chairmqn Robert Dell, former Assistant Attorney General Lois Schiffer and Frederic Kellogg, who was an adviser to Attorney General Elliot Richardson.

Harvard Law professor Laurence Tribe is also among the signatories. In a phone interview Tuesday, he described McConnell's efforts as an attempt to turn the Senate trial into a "political whitewash."

"The reason the Framers gave the Senate the sole power to try impeachments, rather than conducting merely a poll of some political kind, is that they contempted that there would be evidence, there would be witnesses," Tribe, who advised House Democrats during the impeachment proceedings, said.

He noted that senators have to take an additional oath ahead of a Senate impeachment proceeding, and said the country "is entitled" to each senator having "an open mind and try to get to the truth."

However, Tribe predicted that not all is lost when it comes to getting witness testimony in the Senate trial. He said that after some Republicans such as Sen. Mitt Romney said they'd be interested in hearing from Bolton, pressure may mount during the trial to start calling in witnesses.

"We shouldn't assume at this point that McConnell is going to be able to conduct a completely political whitewash, although that's certainly where this is leading," Tribe said.

It's unclear exactly when the Senate impeachment trial will begin. The House voted to impeach Trump for abuse of power and obstruction of Congress largely along party lines last month, but Speaker Nancy Pelosi has not yet transmitted the articles to the Senate.

Rather, she said the House would keep them as it sees which rules the Senate adopted, an idea that Tribe also promoted on Twitter and through an op-ed in The Washington Post.

Tribe said Tuesday that he is in touch with Pelosi as well as other top House Democrats, and that Senate Democrats have also sought his guidance ahead of the impeachment trial. He declined to provide details on the Senate briefings.