Meanwhile, Republicans remain largely silent.
Congressional Democrats on Monday pressed their demands for full disclosure of a whistleblower's complaint about Donald Trump and intensified their calls for impeachment. Trump insisted anew he did nothing wrong in a conversation with Ukraine's leader that is at the center of the complaint.
Republicans remained largely silent amid reports that the president pressured Ukraine's leader to help investigate political rival Joe Biden at the same time the White House was withholding $250 million in aid to the Eastern European nation.
Trump acknowledged Monday that he didn't want to give money to Ukraine if there were corruption issues. His comments raised further questions about whether he improperly used his office to pressure the country into investigating the former vice president and his family.
"It's very important to talk about corruption," Trump told reporters as he opened meetings at the United Nations. "If you don't talk about corruption, why would you give money to a country that you think is, is corrupt?"
Trump has sought, without evidence, to implicate Biden and his son Hunter in the kind of corruption that has long plagued Ukraine. Hunter Biden worked for a Ukrainian gas company at the same time his father was leading the Obama administration's diplomatic dealings with Kyiv. Though the timing raised concerns among anti-corruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.
The matter is under new scrutiny following the whistleblower's mid-August complaint, which followed Trump's July 25 call with Ukraine President Volodymyr Zelenskiy. The person who filed the complaint did not have firsthand knowledge of the call, according to a person familiar with the matter.
Lawmakers are demanding details of the complaint, but the acting director of national intelligence has refused to share that information, citing presidential privilege.
Senate Democratic Leader Chuck Schumer of New York on Monday called on Majority Leader Mitch McConnell to investigate the whistleblower's complaint. In a letter to McConnell, he said that the Republicans' "see no evil, hear no evil" attitude toward the president's actions "is unacceptable and must change."
"In the face of this dire warning and the Trump administration's effort to cover it up, the Republican-led Senate has remained silent and submissive, shying away from this institution's constitutional obligation to conduct oversight," Schumer wrote in the letter.
House Speaker Nancy Pelosi said Sunday that unless the administration provides more information to Congress, its officials "will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation."
A person familiar with the matter has told The Associated Press that Trump urged Zelenskiy to investigate Hunter Biden. The person wasn't authorized to discuss the issue publicly and spoke on the condition of anonymity.
Schumer called on McConnell to take five specific steps to probe the current situation, including issuing a subpoena to compel the whistleblower's complaint to be delivered to Congress. He said Republicans should tell the White House to release transcripts of Trump's conversation with the Ukraine president and identify who in the White House sought to delay $250 million in aid to Ukraine.
Trump and Zelenskiy plan to meet on the sidelines of the U.N. General Assembly this week.