COLUMBUS, Ohio — A bill imposing one of the most stringent abortion restrictions in the nation was signed into law in Ohio on Thursday, banning abortions after a detectable heartbeat in a long-sought victory for abortion opponents that drew an immediate constitutional challenge.

In signing the heartbeat bill, Republican Gov. Mike DeWine broke with his predecessor, Republican John Kasich, who had vetoed the measure twice on grounds that it was unconstitutional.

But DeWine defended Ohio Republicans’ decision to push the boundaries of the law, because “it is the right thing to do.”

“Taking this action really is a kind of a time-honored tradition, the constitutional tradition of making a good faith argument for modification or reversal of existing legal precedents,” he said. “So that is what this is.”

He said it’s the government’s job to protect the vulnerable. The bill outlaws abortions once a fetal heartbeat is detected, which doctors say can be as early as five weeks into pregnancy, before many women know they are pregnant.

Ohio’s closely divided politics had slowed the progress of the bill as it has caught momentum elsewhere , forcing years of debate in the state where the movement originated. Of five previous states that have passed heartbeat bills, three have seen their laws struck down or blocked by the courts, another faces a legal injunction and the fifth is awaiting governor’s action.

DeWine’s action came a day after the latest version of the bill cleared the Republican-controlled Legislature.

Even before the bill was signed, the ACLU of Ohio said it was preparing a constitutional challenge to the law on behalf of Pre-Term Cleveland and three other Ohio abortion clinics.