Top Democrats on Sunday renewed their demands for witnesses to testify at President Trump’s impeachment trial, citing newly released emails showing that the White House asked officials to keep quiet over the suspension of military aid to Ukraine just 90 minutes after Mr. Trump leaned on that country’s president to investigate former Vice President Joseph R. Biden Jr.

The emails, released late Friday by the Trump administration to the Center for Public Integrity, shed new light on Mr. Trump’s effort to solicit Ukraine to help him win re-election in 2020, the matter at the heart of the House’s vote on Wednesday to impeach him for “high crimes and misdemeanors.”

With the Senate’s Democratic and Republican leaders at odds over the trial’s format, Democrats seized on the emails in an effort to put pressure on Senator Mitch McConnell of Kentucky, the majority leader. Mr. McConnell, who wants a bare-bones proceeding, has rejected a proposal by his Democratic counterpart, Senator Chuck Schumer of New York, to have four top White House officials testify.

One of those officials is Michael Duffey, a senior budget official who told the Pentagon to keep quiet about the aid freeze because of the “sensitive nature of the request,” according to an email sent on July 25. An hour and a half earlier that day, Mr. Trump asked President Volodymyr Zelensky of Ukraine to “do us a favor, though” and investigate Mr. Biden and his son Hunter Biden.