[Read about states rushing to restrict abortion, or to protect it.]

Missouri has a long history of anti-abortion activism. About six of the roughly two-dozen abortion cases that have been heard by the Supreme Court have come from the state, according to Samuel Lee, a lobbyist for anti-abortion bills who has been working on the issue in the state since the late 1970s. Even Democratic lawmakers have supported such bills in past decades, because of a strong Catholic contingent in the party. In the mid-1970s, a Democratic state senator, Lawrence Lee, called for a constitutional convention to block abortion.

The current bill’s passage in the Senate was bumpy. Democrats filibustered for several hours, then there was a recess. For a while, it almost seemed like passage might not happen. But senators came back to the floor around 3 a.m., said Samuel Lee, who was in a side gallery at the time, and passed it with little debate.

“It was a relief,” Mr. Lee said.

Even if the bill is signed into law, that does not mean it will go into effect. Heartbeat bills have passed, and been suspended in court challenges, in several states. (Many states say heartbeat bills would prohibit abortion at around six weeks; Missouri’s bill estimated that it would be at around eight weeks.) Two bans, in Iowa and North Dakota, died in court challenges before this year, and Kentucky’s bill, which passed this year, was suspended by a judge.