WASHINGTON — Senate Republicans, who for seven years have promised a repeal of the Affordable Care Act, took a major step on Thursday toward that goal, unveiling a bill to make deep cuts in Medicaid and end the law’s mandate that most Americans have health insurance.

The 142-page bill would create a new system of federal tax credits to help people buy health insurance, while offering states the ability to drop many of the benefits required by the Affordable Care Act, like maternity care, emergency services and mental health treatment.

But the measure landed in rough seas ahead of a vote that Senator Mitch McConnell of Kentucky, the majority leader, wants next week. Four conservative senators, Rand Paul of Kentucky, Ted Cruz of Texas, Mike Lee of Utah and Ron Johnson of Wisconsin, announced that they would oppose it without changes — more than enough to bring it down.

“It does not appear this draft as written will accomplish the most important promise that we made to Americans: to repeal Obamacare and lower their health care costs,” the four wrote in a joint statement.