WASHINGTON — Several Republican senators on Monday proposed a partial replacement for the Affordable Care Act that would allow states to continue operating under the law if they choose, a proposal meant to appeal to critics and supporters of former President Barack Obama’s signature health law.

But the plan was attacked by Democrats as a step back from the Affordable Care Act’s protections, and it was unlikely to win acceptance from conservative Republicans who want to get rid of the law and its tax increases as soon as possible. If anything, the proposal — by Senators Bill Cassidy of Louisiana, a medical doctor, and Susan Collins of Maine, a moderate Republican — may show how difficult it will be for Republicans to enact a replacement for the Affordable Care Act.

Legislation that can pass muster in the more conservative House may not win enough support in the Senate. A bill with broad appeal in the Senate may fail in the House.

Under the proposal, states could stay with the Affordable Care Act, or they could receive a similar amount of federal money, which consumers could use to pay for medical care and health insurance. “We are moving the locus of repeal to state government,” Mr. Cassidy said. “States should have the right to choose.”