WASHINGTON — The Senate on Tuesday afternoon rejected a Democratic attempt to subpoena the White House for documents related to the Trump administration's effort to hold and then release military aid for Ukraine — the centerpiece of the impeachment trial underway.

The effort came as Republicans backed off a plan to restrict the arguments from both House Democrats and President Donald Trump's lawyers to four days instead of six, a retreat that Democrats hoped would signal weakness in the GOP stronghold on the trial's blueprint.

But the Democrats' motion to issue a subpoena was dispensed with, 53-47, with all Democrats voting in support of accepting it and all Republicans against. The effort is the first in a series of Democratic amendments expected Tuesday. By the end of the day, the Senate is expected to approve a Republican effort to set up the rules governing the trial, including a plan to delay any decision on witnesses until after opening arguments and senators' questions.

The starkly partisan vote offered a glimpse of what has already become an extremely divisive Senate impeachment trial. Senators of both parties clung closely to their sides' talking points. Lawyers representing the president dubbed the impeachment effort "ridiculous." Rep. Adam B. Schiff, a California Democrat serving as the lead impeachment manager, called it "ass-backwards ... to have a trial and then ask for witnesses," as Republicans plan to do.

Although Republicans plan to punt a debate over witnesses until after both sides present their arguments, Republicans did back down on an effort to fast-track the proceedings. Senate Majority Leader Mitch McConnell, R-Ky., wanted to limit the arguments to 24 hours over two days for each side, creating extremely long days. Instead, each side will have 24 hours over three days. The change was announced Tuesday afternoon as the clerk of the Senate read the resolution aloud at the start of a debate over whether witnesses and documents would be subpoenaed.

The change was made after a discussion at a closed-door GOP lunch Tuesday, according to a McConnell aide. Some Republicans raised concerns about the original timeline. With a narrow 53-seat majority in the Senate, McConnell has little wiggle room to keep his caucus united, and he has boasted that he has the GOP votes to pass the rules package without any Democratic support.

Sen. Susan Collins, R-Maine, "and others raised concerns about the 24 hours of opening statements in two days and the admission of the House transcript in the record," according to Collins spokeswoman Annie Clark. "Her position has been that the trial should follow the Clinton model as much as possible. She thinks these changes are a significant improvement."

McConnell has said repeatedly that his trial plan was based on the one used during President Bill Clinton's impeachment, which included 24 hours over three days for each side as well as admission of the House records.

Democrats have also blasted the idea of holding 12 hours of opening arguments daily spread over four days, arguing that McConnell was trying to ensure portions of the trial were conducted in the dark of night and completed as quickly as possible.

It also would have meant extraordinarily long sessions that might not have ended until 3 a.m., after including breaks and meals for senators.

Some Republicans appeared ready to support the resolution despite the long days, with none publicly announcing they would reject the earlier timetable.

An hour before the change was read on the Senate floor, Sen. Mitt Romney of Utah, viewed as perhaps the Republican most likely to support the call for witnesses, defended the resolution. "We're going to listen to it whether we hear it in 12-hour blocks or eight-hour blocks," he told reporters.

The new timeline would push the trial — which could have wrapped up by early next week — at least into the end of next week.

The updated resolution also automatically accepts the House's record of evidence in its impeachment inquiry, which was not done in the original version of the resolution.