WASHINGTON — An Obama administration proposal to reduce Medicare payments for many prescription drugs has run into sharp bipartisan criticism, suggesting that it is easier to diagnose the problem of high prices than to solve it.

Patients’ advocates have joined doctors and drug companies in warning that the federal plan could jeopardize access to important medications. Every member of the Senate Finance Committee — 14 Republicans and 12 Democrats — and more than 300 House members have expressed concern.