If Senate Democrats have their way, Supreme Court nominee Merrick B. Garland would sit for confirmation hearings starting in late April and receive an up-or-down vote on the Senate floor about a month later, just before Memorial Day. That is the timeline proposed in a letter sent Monday to Republican leaders by Democratic members of the Senate Judiciary Committee.

The Democrats, of course, are exceedingly unlikely to have their way, because the Republican leaders they are addressing — Majority Leader Mitch McConnell (Ky.) and Judiciary Committee Chairman Charles E. Grassley (Iowa) — have made clear that they will not act on any Supreme Court nomination made by President Obama in this election year.

Still, the letter stands to figure in the standoff over Garland’s confirmation by establishing a baseline for the Senate’s typical handling of a high-court nomination. Attached to the letter is an analysis of the average timing of action on nominees from 1975 until the present. According to those standards, the Democrats claim, hearings on Garland should begin on April 27, with a committee vote roughly two weeks after that, and an up-or-down floor vote on May 25.

“Your refusal to consider his nomination would be an unprecedented break from the Senate’s history and would harm our constitutional democracy and undermine the Supreme Court’s ability to be our nation’s final arbiter of the law,” the letter says, echoing arguments made by many Democrats — including by Vice President Biden in a speech delivered to Georgetown University law students last week.

The Democrats, led by ranking Judiciary member Patrick J. Leahy (Vt.), calculated that since John Paul Stevens was confirmed to the court in 1975, nominees have waited on average 42 days for a hearing, 57 days for a committee vote and 70 days for a final floor vote.

Republicans have wielded precedents of their own — none more frequently than Biden’s own words from 1992 in which he suggested that he would not take action on a nominee made in “the full throes of a presidential election year.” McConnell has frequently mentioned that the Senate has not taken action on a presidential-election-year nomination made by a president of the opposite party since Grover Cleveland was in the White House.

Asked about the letter, McConnell spokesman Don Stewart quoted the 2005 words of Senate Minority Leader Harry M. Reid: “Nowhere in [the Constitution] does it say the Senate has a duty to give presidential nominees a vote. It says appointments shall be made with the advice and consent of the Senate. That is very different than saying every nominee receives a vote.”

Grassley, meanwhile, sought Monday to rebut the suggestion that he adopted the hardline position against Garland’s confirmation under pressure from McConnell.

At a town hall meeting Monday in Rock Rapids, Iowa, he told the audience he decided he would not hold hearings on any Obama nominee on the day Justice Antonin Scalia died — without consulting with McConnell.

“I want to clear up that I did not have any conversation with Mitch McConnell,” Grassley said. “For a whole week, I didn’t know that my position would be agreed with except I suppose Mitch McConnell.”

Grassley told a Des Moines Register reporter in the hours after Scalia’s death was reported that he had no comment on whether the Judiciary Committee would hold hearings, only to issue a written statement hours later indicating his opposition to confirming an Obama nominee.

Grassley reiterated that he plans to stick with his position of not allowing hearings on any nominee this year in order to ensure that the Supreme Court does not get dragged into the political spectacle of the 2016 presidential election.

“We know that his hearing is going to be strictly political,” Grassley said. “Why spend time on theater that won’t produce anything?”

Garland is continuing to meet individually with senators this week during the congressional recess. On Tuesday afternoon, he is set to meet with his first Republican, Sen. Mark Kirk of Illinois.

Snell reported from Rock Rapids, Iowa.