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

Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleyHillicon Valley: Facebook, Google struggle to block terrorist content | Cambridge Analytica declares bankruptcy in US | Company exposed phone location data | Apple starts paying back taxes to Ireland Overnight Energy: Pruitt taps man behind 'lock her up' chant for EPA office | Watchdog to review EPA email policies | Three Republicans join climate caucus Watchdog to probe EPA email preservation 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 UdallThomas (Tom) Stewart UdallOvernight Energy: Pruitt taps man behind 'lock her up' chant for EPA office | Watchdog to review EPA email policies | Three Republicans join climate caucus Pruitt hires outside attorney as investigations mount: report Overnight Energy: Pruitt gets Senate grilling | Dems want investigation into Pruitt's security chief | Interior officers arrested 13 in border surge | Advisers pan science 'transparency' plan 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 AlexanderAndrew (Lamar) Lamar AlexanderGOP lawmakers want Trump to stop bashing Congress The Hill's Morning Report - Sponsored by CVS Health - A pivotal day for House Republicans on immigration GOP, Dem lawmakers come together for McCain documentary MORE (R-Tenn.) will lay out the GOP position on filibuster reform later Tuesday when he speaks at the Heritage Foundation.