Utah's state Senate has voted unanimously to decriminalise polygamy among consenting adults.
Under current laws, anyone found to have multiple spouses could face up to five years in prison.
Proponents of the bill argue it would remove the secrecy surrounding communities which practise polygamy and allow victims to report abuse. But critics warn it could empower abusers.
The bill still needs to be approved by the state's House of Representatives.
Under the proposal, having more than one spouse - or bigamy - would be considered a misdemeanour, on a par with a traffic violation.
Mores serious crimes, such as coerced bigamy, would be treated as a felony.
About 30,000 people are thought to live in polygamous communities in Utah, according to the Associated Press.
The state is home to the Church of Jesus Christ of Latter-day Saints, also known as the Mormon Church, which accepted polygamy in its early history before banning it in 1890.
It now excommunicates anyone who practises polygamy but a number of fundamentalist Mormon offshoots continue to do so.
While plural marriage is illegal under Utah's constitution, officials in the state have largely chosen not to prosecute those involved.
In 2001, polygamist Tom Green became the first man to be convicted of bigamy in more than half a century over his marriage to five women. He was later also sentenced for child rape after one of his wives became pregnant when she was 13 years old.