WASHINGTON — The high-profile fight over potentially dramatic witness testimony at an impeachment trial of President Donald Trump has overshadowed the Senate's possible demand for a different type of revealing cache of new evidence — withheld documents.

Senate Democrats have pushed to include in the trial documents that the Trump administration refused to turn over during the House investigation. But they need at least four Republicans to vote with all Democrats and independents for the Senate to subpoena witnesses or documents, and it's not clear they have those votes.

The trial is expected to begin next week, following Wednesday's House vote to transmit the articles of impeachment.

Speaker Nancy Pelosi, D-Calif., says the House issued five subpoenas and made 71 requests and got none of the information.

Documents generally aren't the most thrilling part of a trial, and don't produce nearly as many made-for-television moments as live witness testimony.

But, at times, what lawyers and investigators dig up in documents can make a case.

"Often documents speak more loudly than witnesses," Sen. Chris Coons, D-Del., said.

Sen. Richard Blumenthal, D-Conn., a former state attorney general who has argued before the Supreme Court, said those unfilled requests from House Democrats include documents from numerous agencies including the Defense, State and Energy departments.

"And these documents tell a story. Documents don't lie," Blumenthal said. "Documents are some of the most persuasive evidence that can be produced, and here it can corroborate the powerful evidence we already have from witnesses."

Blumenthal said he has been thinking ahead to issues that will come up during the trial. There are documents, he said, Trump's defense team could bring up as well as those that could be helpful to the House managers presenting the case for impeachment.

Blumenthal noted that there are emails, instant messages, and one ambassador's notes of conversations with Gordon Sondland, the ambassador to the European Union, that could further show how Trump handled the withholding of $391 million in military aid to Ukraine, which is at the center of the impeachment case.

"We know these documents exist," Blumenthal said. "I don't know that there'll be a smoking gun, but they'll offer critical corroboration."

Some moderates such as Sen. Susan Collins of Maine are reportedly working to establish a group of Republicans who would vote that way. But the majority of Senate Republicans are likely to oppose the effort.

Judiciary Chairman Lindsey Graham, R-S.C., said he wouldn't vote for the Senate to subpoena documents, in part because the House chose to send their case over quickly, and argued it was too urgent to go through court fights for witnesses and documents.

"Until you can show me something else, then I'm not very interested," Graham said. "I am ready to base my vote on what they did."

Minority Leader Charles E. Schumer, D-N.Y., repeatedly has said he will call for votes during the trial on whether the Senate should subpoena three specific sets of documents, which he said would be "narrowly drawn" and limited "to electronic communications, memoranda and related records of the relevant senior officials in the White House, Office of Management and Budget, and Department of State."

"Our understanding is that these records have already been collected by the White House counsel and counsel in the relevant agencies, so production in response to the Senate subpoenas should be neither burdensome nor time-consuming," Schumer said in a December letter to Majority Leader Mitch McConnell, R-Ky.

But McConnell has resisted such efforts. And legal fights over public records requests from media and advocacy organizations show not only the potential information in the documents, but also how aggressively the Trump administration could fight disclosure of the records at an impeachment trial.

In one case brought by The New York Times, the Office of Management and Budget said it would not turn over any of the 40 pages of emails between a top White House aide and OMB official Michael Duffey. Those emails discuss the holdup of military aid to Ukraine.

House Democrats argue that Trump abused the power of his office by withholding that congressionally approved aid in exchange for the Ukrainian president announcing an investigation into former Vice President Joe Biden, a top contender for the 2020 Democratic presidential nomination.

OMB told The Times that releasing emails that involve the president's staff and internal policy deliberations falls under a public records law exemption, and would "inhibit the frank and candid exchange of views that is necessary for effective government decision-making."

In another case, a federal judge ordered the Defense Department and OMB to turn over documents to the Center for Public Integrity about the Ukraine aid.

Many of those emails were partially or fully redacted by the Trump administration when turned over in December, including an Aug. 30 email from Duffey to Elaine McCusker, the Pentagon comptroller.

But Just Security reported this month that, according to the unredacted email, Duffey told McCusker that Trump had directed the aid withheld.

"Clear direction from POTUS to continue to hold," Just Security reported that Duffey wrote in the email.

Sen. Patrick J. Leahy, D-Vt., a former prosecutor, pointed to those recent revelations through news reporting and public records request lawsuits from media and advocacy organizations.

"If they're holding that back, as explosive as they are, you wonder how much else they've held back," Leahy said. "And they were made before anybody was thinking about how do we protect ourselves from impeachment."