President Donald Trump talks with reporters on the South Lawn of the White House in Washington, Sunday, Sept. 22, 2019, as he prepares to board Marine One for the short trip to Andrews Air Force Base. Trump is traveling to Texas and Ohio before heading to New York for the upcoming United Nations General assembly. (AP Photo/Susan Walsh)
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PHOTO: Susan Walsh/AP
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(CNN) —  

Senate Democratic leader Chuck Schumer sought to turn up the pressure Monday on Senate Majority Leader Mitch McConnell and Senate Republicans by outlining a plan for how he wants Republicans to investigate a whistleblower complaint that has sent shockwaves through Washington.

In a letter to McConnell on Monday, Schumer writes that he is calling on the majority leader and Senate Republicans who control the chamber to “take immediate action to stop President Trump from withholding an Intelligence Community whistleblower complaint that by law must be transmitted to Congress, and to begin an investigation into the Administration’s handling of security assistance to Ukraine.”

Schumer calls for hearings “to determine exactly what prompted the whistleblower to file this urgent complaint,” and says they should feature testimony from a host of administration officials, including acting Director of National Intelligence Joseph Maguire and Secretary of State Mike Pompeo. Schumer also calls for testimony from Trump’s private attorney Rudy Giuliani.

Schumer also states that a subpoena should be issued “to compel the delivery of the whistleblower complaint to Congress, as required by law.”

The letter is the latest effort by congressional Democratic leaders to push for an investigation into the whistleblower complaint amid a controversy surrounding Trump and his contact with Ukraine.

McConnell addressed the whistleblower complaint in remarks on the Senate floor on Monday afternoon and accused Schumer of politicizing the issue.

“The Senate Select Committee on Intelligence has long worked on a bipartisan basis in secure settings out of the public spotlight, to conduct critically important oversight of classified and sensitive matters,” the Senate GOP leader said. “So, I’ve been disappointed to see our colleague, the Democratic leader, choose to politicize the committee’s ongoing efforts with respect to a recent whistleblower allegation, the specific subject of which is still unknown.”

McConnell went on to say that “Chairman Burr and Vice chairman Warner have been working together to get the acting director of intelligence and the intelligence community’s inspector general before the committee this week to discuss the matter,” referring to Republican Sen. Richard Burr, the chairman of the Senate Intelligence Committee and Sen. Mark Warner, the top Democrat on the committee.

McConnell said that it is important that the work be handled “in a secure setting” and in a “bipartisan fashion.”

Trump acknowledged on Sunday that he discussed former Vice President Joe Biden in a July call with Ukraine’s president. CNN has previously reported that Trump pressed Ukrainian President Volodymyr Zelensky in the call to investigate Biden’s son, Hunter, according to a person familiar with the situation. That call was also part of the whistleblower complaint submitted to the Intelligence Community Inspector General. There is no evidence of wrongdoing by Biden or his son Hunter.

Schumer’s letter calls on Senate Republicans to demand that the White House release the transcript of the conversation between Trump and the Ukrainian President. It also asks that Trump administration officials be identified “who directed that $341 million of security assistance to Ukraine be delayed.”

The letter from the Senate Democratic leader comes after House Speaker Nancy Pelosi called over the weekend for the whistleblower to come before Congress.

Maguire is set to testify in open session before the House intelligence committee on Thursday.

In a letter to House Democrats, Pelosi wrote, “At that time, we expect him to obey the law and turn over the whistleblower’s full complaint to the Committee. We also expect that he will establish a path for the whistleblower to speak directly to the House and Senate Intelligence Committees as required by law.”

Pelosi warned in the letter, however, that “If the Administration persists in blocking this whistleblower from disclosing to Congress a serious possible breach of constitutional duties by the President, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation.”

More senators weigh in on whistleblower controversy

Republican Sen. Chuck Grassley of Iowa, the chairman of the Senate Finance Committee, told CNN on Monday that he hopes that he will have a chance to speak with the whistleblower, but would not weigh in on Trump’s contacts with the Ukrainian President.

“I would like the whistleblower to come and talk to me so I know what his story is,” Grassley said when asked if the complaint should be turned over to Congress. “I don’t want to hear it second hand.”

When asked if it is an appropriate use of Trump’s office to talk to Ukraine’s president about the Bidens, the GOP senator sidestepped the question, saying, “I think through the news media maybe you can get the whistleblower to come and talk to us.”

Warner would not say if he believes that it’s an impeachable offense if Trump urged Ukraine to probe Biden in exchange for releasing military aid.

“What I want to do is get the truth first,” Warner said, adding that he wants a probe done in a bipartisan way. “We need to get the facts.”

This story has been updated with additional developments Monday.

CNN’s Ted Barrett, Sam Fossum and Devan Cole contributed to this report.