Senate Democrats said leaked information from former national security adviser John Bolton’s forthcoming book will lead to the Senate summoning him and other Trump administration officials to testify at the impeachment trial.

“This is stunning,” Senate Minority Leader Chuck Schumer, a New York Democrat, said Monday. “It goes right to the heart of the impeachment charges.”

The Senate is expected to vote this week on whether to call witnesses.

Two Republicans, Susan Collins of Maine and Mitt Romney of Utah, have said they are inclined to call witnesses.

“There is increased pressure,” Schumer said. “The eyes of America are on the Republicans in the Senate."

But four Republicans would have to vote with all Democrats to provide the 51 vote majority to call witnesses.

“It’s gut-check time for members of the United States Senate,” Sen. Tammy Baldwin, a Wisconsin Democrat, told reporters Monday.

The draft of Bolton’s book was leaked in part Sunday, revealing Bolton’s claim that President Trump told him to block security aid to Ukraine until the government promised to investigate former Vice President Joe Biden and Democratic Party officials.

Schumer added, "It's an uphill fight," to call witnesses, "but every few days something else happens that makes it clearer and clearer we need the truth."

Top Senate Republicans said if Bolton and other witnesses sought by Democrats are called, Trump’s defense team should also have the right to call witnesses. The list is likely to include Hunter Biden, son of Joe Biden, who took a lucrative job with a Ukrainian gas company while his father was the country’s point person for the Obama administration.

“If there is a desire and decision by the Senate to call Democratic witnesses, then at a minimum the Senate should allow President Trump to call all relevant witnesses he has requested,” Senate Judiciary Committee Chairman Lindsey Graham of South Carolina, said on Twitter.

Democrats said the GOP will have a hard time denying that Bolton’s claims show direct evidence the president engaged in behavior that led to the abuse of power impeachment article.

Democrats say the president abused the power of his office by withholding the Ukraine aid to pressure the government to conduct the investigations.

“There is now a very strong reason for them to vote to call witnesses,” Sen. Ben Cardin, a Maryland Democrat, said.