All Senate Republicans wrote Majority Leader Harry Reid (D-Nev.) on Wednesday, vowing to oppose any policy-driven changes to the healthcare bill in the Senate.

All 41 GOP senators signed a letter to Reid pledging to uphold the "Byrd rule," which requires that all elements of a bill passed using budget reconciliation rules be strictly related to the budget.

The GOPers' pledge amounts to a threat to block any changes sought by Senate Democrats on issues like abortion or immigration, or perhaps the reintroduction of the public option to the healthcare bill.

"We wish to inform you that we will oppose efforts to waive the so-called Byrd Rule during Senate consideration of any reconciliation bill concerning health reform," the senators wrote. "As it takes 60 votes to waive the Byrd Rule, we can ensure that any provision that trips the Byrd Rule will be stripped from the bill, which will require that the bill be sent back to the House for further consideration and additional votes."

The letter has the effect of putting Reid and Democrats on notice that any attempt to go beyond the scope of budget reconciliation rules will be met with fierce GOP opposition.

The reconciliation process is being considered to make changes to the Senate's original healthcare bill in order to placate enough House Democrats' concerns about that bill. The process would allow senators to approve changes with a simple-majority vote, instead of the 60 votes typically needed to end a filibuster.

Republicans have already said they'll oppose that process, and the Senate's second-ranking Republican, Minority Whip Jon Kyl (Ariz.), said Wednesday he thinks the GOP has enough votes to stop some parts of the reconciliation bill.