A day after Democratic lawmakers demanded that former National Security Adviser John Bolton testify in President Donald Trump's impeachment trial, grassroots political action groups urged the American public to call their representatives and add their voices to the call for a fair trial.

"Hearing from first-hand witnesses in the Senate trial is now a necessity," tweeted the progressive group Stand Up America. "Call your senators now and demand a fair trial."

We now know that John Bolton would testify that Trump told him directly that he was soliciting a bribe from Ukraine. Hearing from first-hand witnesses in the Senate trial is now a necessity. Call your senators now and demand a fair trial. Text TRIAL to 21333. #BoltonMustTestify — Stand Up America (@StandUpAmerica) January 27, 2020

The New York Times reported Sunday night that an unpublished manuscript of Bolton's memoir showed that in August 2019 Trump told the then-national security official that he didn't want to lift a freeze on aid to Ukraine until the country's government agreed to launch investigations into Democrats including former Vice President Joe Biden and his son, Hunter.

Senate Minority Leader Chuck Schumer called the news "stunning" Monday morning and demanded Republicans end what Democratic leaders have called a "cover-up" of the facts.

"It goes right to the heart of the charges against the president," Schumer said at a press conference. "Ambassador Bolton essentially confirms the president committed the offenses charged in the first article of impeachment."

"We have a witness with firsthand evidence of the president's actions for which he is on trial," he continued. "How can Senate Republicans not vote to call that witness and request his documents?"

"We have a witness with firsthand evidence of the president's actions for which he is on trial. [Bolton] is ready and willing to testify. How can Senate Republicans not vote to call that witness and request his documents?" —@SenSchumer pic.twitter.com/tPoYqyqBOv — CAP Action (@CAPAction) January 27, 2020

Bolton has previously stated that he would be open to testifying in the impeachment case and has suggested he may have information pertinent to the trial. Senate Majority Leader Mitch McConnell (R-Ky.), however, has coordinated with the White House on his management of the Senate trial and last week led his party through several votes in which the GOP rejected calls for government documents and witnesses to be subpoenaed.

"Blow up the phones, children," actress and activist Sandra Bernhard tweeted after the news of Bolton's unpublished revelations broke.

blow up the phones children! #WitnessesAndDocuments https://t.co/8ztkhiXjNB SCROLL TO CONTINUE WITH CONTENT Never Miss a Beat. Get our best delivered to your inbox.





— Sandra Bernhard (@SandraBernhard) January 27, 2020

Bernhard's tweet was one of many making similar calls, as activists called on the public to contact their senators.

We are citizens of the UNITED States of America, and we must indeed unite together to stop this GOP cover-up. Reach your senator at 855-912-2982 and comment below where you've called from. Let's all hold each other accountable. #AmericansWantWitnesses #CallYourSenators — Karine Jean-Pierre (@K_JeanPierre) January 27, 2020

This is no longer a done deal. You need to call BOTH of your senators NOW, regardless of party to demand that #BoltonMustTestify so that they can gauge overwhelming public support for witness testimony. If the main line is busy, google their local office numbers. (202) 224-3121. — Amee Vanderpool (@girlsreallyrule) January 27, 2020

As part of its lobbying and mobilization effort, MoveOn.org announced Monday it was rolling out a new billboard campaign aimed at key Republican senators.

The billboard was positioned on Capitol Hill and targets Sens. Susan Collins (R-Maine), Lisa Murkowski (R-Alaska), Mitt Romney (R-Utah), and others who have said they would consider having witnesses as part of the impeachment trial.

Republicans "are refusing to hold a fair trial, refusing the Senate access to White House documents and refusing to hear from valuable witnesses," said Rahna Epting, executive director or MoveOn. "This is not the American way. Republicans are showing that they care more about their loyalty to Donald Trump than the interests of the American taxpayer and upholding our constitution."

The few Republicans "who can still choose the Constitution over Trump", Epting said, "need to know that voters all across the country want the full truth—and history will remember if they stand up for the truth."