WASHINGTON—With Republicans struggling to forge a health-care plan that combines low costs with broad coverage, two GOP senators Monday offered an idea that sidesteps that tough trade-off: allowing, but not forcing, states to opt out of major parts of the Affordable Care Act.

Sens. Susan Collins (R., Maine), a centrist, and Bill Cassidy (R., La.), a conservative, hope this approach, embodied in a bill they introduced Monday, can unify congressional Republicans—a task that has proven elusive—in part by embracing a long-held GOP value of returning power to the states.

Democratic leaders immediately said the plan falls far short of a full replacement for the ACA and that it would result in millions losing coverage. And some conservative Republicans, including in the House, may balk at a bill that is less than a complete repeal and would let states like New York and California continue under the ACA.

But Ms. Collins and Mr. Cassidy argued that theirs is the only approach with a chance of attracting support not only from Republicans but from the handful of Democrats needed to enact a complete ACA replacement.

“At some point in this process, we’re going to need a bill that would get 60 votes, OK?” Mr. Cassidy said. “Now, if you can say to a blue-state senator who’s really invested in supporting Obamacare, ‘You can keep Obamacare, but why force it upon us?’, we think that helps us get to 60.”