"I will never stop fighting to preserve this successful program," said Reid. "As long as I am in the Senate, I will oppose Republican plans to weaken or undermine it because Republicans’ plan to end Medicare is one idea whose time will never come."

House Republicans passed a budget for fiscal year 2012 earlier in the year that would cut $5.8 trillion in spending over 10 years and transform Medicare into a kind of voucher system. The GOP blueprint would balance the budget by 2040 without raising taxes. 

Democrats have repeatedly criticized GOP proposals to change Medicare, which Republicans say should be considered in the context of talks to reduce annual deficits and raise the debt ceiling. 

Reid is expected to play a role in talks involving the White House and Speaker John BoehnerJohn Andrew BoehnerFeehery: The next Republican wave is coming Rift widens between business groups and House GOP Juan Williams: Pelosi shows her power MORE (R-Ohio) aimed at raising the debt ceiling by Aug. 2.