A letter sent by 19 conservative activists urging congressional Republicans to pursue a delay of Obamacare highlights a growing dispute over how to oppose the president’s signature healthcare reform legislation.

The letter, sent Tuesday to Speaker of the House John Boehner (R., Ohio) and Senate Minority Leader Mitch McConnell (R., Ky.), stands in contrast to a movement within the Senate urging senators to oppose any continuing resolution that contains funding for Obamacare.

Both the delay and the defund efforts rely on the negotiations over the continuing resolution, which Congress will have to pass by the end of September to keep the government funded. If no resolution is passed, the government will shut down.

"With the clock ticking to open enrollment on Oct. 1, it is abundantly clear to members of the Repeal Coalition that the structure at the heart of PPACA is simply not ready," the letter reads.

"To help the American people avoid getting hit by what Sen. Max Baucus memorably referred to as ‘a huge train wreck coming down,’ we the undersigned urge you to insist on, at minimum as part of any final deal, a one-year delay of all 2014 provisions (including mandates, subsidies, and taxes) in the upcoming CR and in fiscal negotiations with the White House," it continues.

"We wanted to lay out what we thought were the minimums that the American people were hoping for," said Heather Higgins, president of Independent Women’s Voice and founder of the Repeal Coalition.

"Full defunding … is not necessarily what should be the final demand," Higgins said.

Boehner’s office was noncommittal on his strategy for the upcoming continuing resolution negotiations.

"The speaker wants to repeal and replace Obamacare—but no decisions have been made regarding the CR at this point," said Michael Steel, a spokesman for Boehner, in an email.

A Republican Senate leadership aide noted that McConnell tried to bring up for a vote the House’s measure to delay the individual mandate for a year, but the aide did not indicate that McConnell’s office has a specific strategy for the upcoming negotiations.

McConnell did not join 12 of his Senate Republican colleagues in signing a letter opposing any funding measure that continues to support Obamacare.

Sen. Mike Lee (R., Utah) is leading the defunding effort, which has garnered support from Sens. Marco Rubio (R., Fla.) and Ted Cruz (R., Texas), among others.

The defunding effort has also attracted the support of several major conservative groups, including the Club for Growth and Heritage Action.

Signatories of Tuesday’s letter are skeptical of the plan.

"I think that strategy is profoundly misguided," said Avik Roy, a fellow at the Manhattan Institute and a writer for National Review, about the defund strategy. Roy signed the letter urging the GOP to pursue a delay.

Several Republican senators have come out against the effort to defund.

"The strategy that has been laid out is a good way for Republicans to lose the House," Sen. Tom Coburn (R., Okla.) has said.

Coburn said the idea "will not work," while Sen. Richard Burr (R., N.C.) has called the idea "the dumbest idea I’ve ever heard."

"The public is not supportive of shutting down the government in order to defund the law," Roy said of what he believes are political liabilities in the plan.

Cruz has contended that the continuing resolution may be the last chance Republicans have to oppose the law.

Roy contested this idea. A one-year delay is a viable option, Roy said, and it is potentially much more palatable to some Democrats from conservative states, such as Sens. Mark Pryor (Ark.) and Kay Hagan (N.C.), than a full defunding effort might be.

John Kartch, a spokesman for Americans for Tax Reform, called the letter a more prudential approach to opposing Obamacare. (Americans for Tax Reform’s president Grover Norquist signed the letter.)

"In short, it's an exhortation to principle while recognizing the need for "world-as-it-is" flexibility," Kartch wrote in an email. "We are confident that congressional conservatives will do everything within their power to delay, defund, and repeal Obamacare, and we support them in their efforts."

Several parts of the law do not appear to be ready for implementation on the current schedule, exposing the law to fraud and security vulnerabilities, Roy said.

The Inspector General for the Department of Health and Human Services released a report on Friday reporting that the department in charge of implementing major parts of the law was months behind schedule in testing the security of the "data hub." The hub will transfer personal identification information, including social security numbers and yearly tax return information, from multiple federal agencies to the exchanges.