Democrats face delays in their effort to reform Senate rules to weaken the filibuster, a leader of that effort acknowledged Monday night.

Sen. Jeff MerkleyJeff MerkleySenate GOP blocks defense bill, throwing it into limbo Lawmakers call on Olympic committee to press China on human rights abuses Senate Democrats call on Biden to push for COVID-19 vaccine patent waivers at WTO MORE (D-Ore.) said Democrats' attempt to adopt new Senate rules would wait until later in January, when they would try to execute the so-called "constitutional option" to change Senate rules with a simple majority.

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"Right now, the Senate majority leader is planning for us to come in just for a single day this Wednesday and then come back in on the 23rd or 24th," Merkley said on MSNBC.

It is at that point — not on Wednesday, as had been originally thought — that Democrats will attempt to modify Senate rules to weaken the filibuster, one of the principal tools of the minority in the chamber.

Democrats have argued they have the power, under the Constitution, to change the Senate's rules with only a simple majority on the chamber's first day of operations. To execute the plan in later January, they would have to technically extend the current session until later this month, and officially begin work on the next term on Jan. 23 or 24.

"By precedent, by tradition, which weighs heavily in the Senate, and by a certain common sense logic, at the start of a two-year period, you set your rules out at the beginning," said Merkley, who noted the Senate may not formally adjourn before then.

Democrats have complained the GOP has abused the filibuster to obstruct and slow down legislation to the point where every measure needs a supermajority of 60 votes to advance.

"We're, in fact, in a constant state of filibuster," Sen. Tom UdallTom UdallRubio vows to slow-walk Biden's China, Spain ambassador nominees Senate confirms Thomas Nides as US ambassador to Israel Flake, Cindy McCain among latest Biden ambassadors confirmed after delay MORE (D-N.M.), a major proponent of reform, said Monday night on NPR.

Every Democratic senator has signed on to a letter endorsing reform in some form, but concern about the specifics of the reform, and how it could affect the party if it loses the majority, have contributed to pushing back any action.

Udall's proposal would eliminate secret holds on nominations and possibly narrow the opportunities senators would have to wage filibusters. In exchange, the minority party in the Senate — for now, Republicans — might enjoy more opportunities to offer and approve amendments.

Sen. Lamar AlexanderLamar AlexanderMcConnell gets GOP wake-up call The Hill's Morning Report - Presented by Alibaba - Democrats return to disappointment on immigration Authorities link ex-Tennessee governor to killing of Jimmy Hoffa associate MORE (R-Tenn.) will lay out the GOP position on filibuster reform later Tuesday when he speaks at the Heritage Foundation.