Twelve senators sent a letter to Senate Majority Leader Harry Reid (D., Nev.) Thursday afternoon declaring their intention to vote against any bill that contains funding for Obamacare.

The senators "believe the only way to avert disaster is to fully repeal Obamacare and start over with a more sensible, practical approach to reforming our healthcare system," they wrote.

The signatories, led by Sen. Mike Lee (R., Utah), include potential presidential hopefuls Sens. Ted Cruz (R., Texas), Rand Paul (R., Ky.), and Marco Rubio (R., Fla.).

"However, if Democrats will not agree with Republicans that Obamacare must be repealed, perhaps they can at least agree with the president that the law cannot be implemented as written," they wrote. "If the administration will not enforce the law as written, then the American people should not be forced to fund it."

The administration’s decision a few weeks ago to delay several key parts of Obamacare set off a firestorm among conservative groups and raised concerns among health care experts that the law would not be ready in time.

Following the administration’s announcements, the House leadership and all 46 Republican senators wrote letters to President Barack Obama asking that other parts of the law, including the individual mandate, be delayed as well.

The letter marks the first official salvo regarding the upcoming continuing resolution and Obamacare. If Congress does not pass a continuing resolution by the end of September, the federal government will shut down.

Congressional Republicans have used budget negotiations in the past to try to force concessions from Democrats, with mixed results.

The most recent negotiations came over the so-called "fiscal cliff" at the end of last year when tax increases coincided with the mandatory spending cuts known as the sequester.

Republicans ultimately had to accept tax increases on top earners, although the Bush-era temporary tax cuts for most Americans were made permanent.