Missouri and at least 19 other states are considering passing laws that would force people to prove their citizenship before they can vote. These bills are not a sincere effort to prevent noncitizens from voting; that is a made-up problem. The real aim is to reduce turnout by eligible voters. Republicans seem to think that laws of this kind will help them win elections, but burdensome rules like these  and others cropping up around the country  pose a serious threat to democracy and should be stopped.

The Missouri legislature is, as Ian Urbina reported in The Times on Monday, on the verge of passing an amendment to the State Constitution that would require proof of citizenship from anyone registering to vote. In addition to the Missouri amendment, which would require voter approval, Florida, Kansas, South Carolina and other states are considering similar rules.

There is no evidence that voting by noncitizens is a significant problem. Illegal immigrants do their best to remain in the shadows, to avoid attracting government attention and risking deportation. It is hard to imagine that many would walk into a polling place, in the presence of challengers and police, and try to cast a ballot.

There is, however, ample evidence that a requirement of proof of citizenship will keep many eligible voters from voting. Many people do not have birth certificates or other acceptable proof of citizenship, and for some people, that proof is not available. One Missouri voter, Lillie Lewis, said at a news conference last week that officials in Mississippi, where she was born, told her they had no record of her birth.