The letter, organized by Virginia Rep. Scott Taylor and Washington Rep. Dan Newhouse, already has signatories numbering in the 20s, according to a source familiar with the letter, and could reach into the 30s by the time it is sent. Taylor's office did not immediately respond to a request for comment.
The letter originated with the Republican Main Street Caucus and will remain open for signatures with the hope of sending it by next week, according to the office of Illinois Rep. Rodney Davis, who chairs the nascent caucus.
Davis sought to make clear the letter was not meant as an ultimatum to Ryan.
"The letter Main Street members are working on is to show their strong support for solving the DACA issue and that we believe the House should address it now, rather than later," Davis said. "Unlike some of our Democrat counterparts, we are not using this issue to play political games or hold government funding hostage. We simply want a solution to help DACA recipients as soon as possible."
Congressional Hispanic Caucus Chairwoman Michelle Lujan Grisham, a New Mexico Democrat, revealed the work on the letter in a pen-and-pad session with reporters Thursday on Capitol Hill.
Two other GOP sources also confirmed the letter's development to CNN.
Grisham characterized the letter as "telling Ryan, 'You've got to fix this. You've got nine days. What is your plan? What is your path?' "
The "nine days" refers to the December 8 deadline to pass legislation to fund the government. Democrats have said that if Republicans need their votes to pass a government funding bill, which they have in the past, then they need to resolve the situation for the Deferred Action for Childhood Arrivals program, which President Donald Trump is ending, by the end of the year.
While Davis denied a spending connection, those closely watching the letter note that the number of signatories -- potentially more than enough to deny Republicans a majority vote without Democratic support -- still sends a warning signal to the speaker about his ability to muster votes.
Some Republicans, including Senate Majority Whip John Cornyn of Texas, have said any DACA deal should not be included in year-end spending legislation. They have not ruled out, though, the possibility of timing a vote on a DACA deal with one on spending legislation.
On Thursday, Ryan did not rule out merging the issues when asked about it at a news conference, but he did urge Democrats to "get to the table."
Grisham referenced a Democratic-led discharge petition to force a vote on one legislative proposal, the Dream Act, which two Republicans have signed and which needs only 22 more members to support it to force a vote on the floor, though the letter does not threaten that its signatories will back the bill, according to one of the sources.
The letter's signatories include members who have long pushed for a DACA fix and some who have been less vocal.
According to one of the GOP sources, the letter tells Ryan that the group would like DACA resolved this year and warns that while they agree a legislative solution should include border security, it should not contain measures sought by members like Republican Rep. Bob Goodlatte of Virginia and the White House. Some of those measures include cuts or changes to overall legal immigration, mandatory workforce verification and hardline enforcement measures.
The letter has come together quickly, mostly this week.