Former President Donald Trump's legal team filed a 78-page trial brief Monday morning, ahead of the Senate impeachment trial. The brief asks the Senate to dismiss the charges, stating their belief that holding a trial is unconstitutional in the first place and also asserting that Trump was engaging in political speech protected by the First Amendment.

"His political speech falls squarely within the protections of the First Amendment under clear Supreme Court precedent (as fully discussed below), and he thus cannot be convicted by a Senate sworn to uphold the Constitution," Trump's lawyers wrote in the brief.

House impeachment managers promptly filed a five-page response to the brief and called his free speech argument "utterly baseless."

"The evidence of President Trump's conduct is overwhelming. He has no valid excuse or defense for his actions," the impeachment managers wrote. "And his efforts to escape accountability are entirely unavailing."

-ABC News' Katherine Faulders

Sen. Richard Shelby, R-Ala., announced in a statement that he will not seek re-election in 2022.

"Today I announce that I will not seek a seventh term in the United State Senate in 2022," Shelby says in the statement. "For everything, there is a season."

He cited no specific reason for his decision not to run.

Shelby is currently the vice chairman and leading Republican on the Senate Appropriations Committee, where he recently served as chairman of the full committee and its subcommittee on defense. He previously served as chairman of the Senate Committee on Rules and Administration and the Senate Banking, Housing, and Urban Affairs Committee, of which he is the longest serving member in history, according to his statement.

Before being elected to the Senate in 1986, Shelby served four terms in the U.S. House representing Alabama's 7th Congressional District. He also served in the Alabama State Legislature.

"Although I plan to retire, I am not leaving today. I have two good years remaining to continue my work in Washington. I have the vision and the energy to give it my all," he said in his statement.

-ABC News' Allison Pecorin

Upon arriving back at the White House on Monday morning after his weekend in Delaware, President Biden was asked about whether former President Trump should lose his "political rights" and be barred from running for office as his second impeachment trial is set to get underway Tuesday.

"He got an offer to come and testify, he decided not to. Let the senate work that out,” Biden said.

House impeachment managers had asked Trump to testify under oath in his upcoming impeachment trial about his conduct on Jan. 6, according to a letter sent last week by lead impeachment manager Rep. Jamie Raskin.

Trump's legal team responded with a letter and rejected the idea of the former president appearing.

In the short response, Trump's attorneys called the request a "public relations stunt."

“The use of our Constitution to bring a purported impeachment proceeding is much too serious to play these games," wrote Trump attorneys Bruce Castor and David Schoen.

While the letter did not answer directly the question of whether Trump will testify, Trump spokesman Jason Miller told ABC News' Chief Washington Correspondent Jonathan Karl, “The president will not testify in an unconstitutional proceeding.”

-ABC News' Molly Nagle, Katherine Faulders and Ben Siegel

The House Ways and Means Committee on Monday is expected to lay out a proposal to send $3,600 per child to millions of American families, as House Democrats work to assemble the $1.9 trillion COVID-19 relief package proposed by President Biden.

The 22-page proposal, first obtained by the Washington Post and confirmed by ABC News, would send $3,600 per child under 6 years old to American families, and $3,000 per child between the ages of 6 and 17. The benefit would decrease for Americans making more than $75,000 annually, or couples earning more than $150,000 a year.

The program would be administered by the Internal Revenue Service. The payments, which would start going out in July, would follow through on the Biden administration's call to expand the Child Tax Credit.

“The pandemic is driving families deeper and deeper into poverty, and it’s devastating. We are making the Child Tax Credit more generous, more accessible, and by paying it out monthly, this money is going to be the difference in a roof over someone’s head or food on their table," Chairman Richard Neal, D-Mass., said in a statement.

Neal's committee is just one of many in the House working on the COVID-19 relief package. The panel is expected to release its full slate of tax-related proposals for the stimulus bill later Monday.

The committees' work will be combined before any vote on the House floor -- and it's unclear if this specific proposal will pass muster with the Senate parliamentarian under the strict rules of the process Democrats are using to pass the package with 50 votes in the Senate.

A Columbia University study of Biden's proposal found that it could cut child poverty in half, and impact more than 5 million American children under 18.

-ABC News' Ben Siegel