MONTPELIER, Vt. — A Republican state senator who’s one of the Legislature’s most outspoken conservatives was arrested Thursday and will face charges of sexual assault, human trafficking and prohibited acts, state police said.

Norman McAllister, who’s in his second two-year Senate term representing Franklin County, was to be arraigned Friday, police said without elaborating on the allegations against him.

McAllister was held Thursday night at a jail in St. Albans on $20,000 bail, police said.

The case was investigated by the Northwest Unit for Special Investigations, a law enforcement team that investigates sex crimes and child abuse.

McAllister, a successful Franklin County dairy farmer, is one of nine Republicans in the 30-member Senate. He has sponsored legislation to require recipients of public assistance to undergo drug testing, a measure given little chance of passage in the current Legislature.

Before his arrest Thursday, he had spent the day at the Statehouse, debating issues including a bill on school governance.

Democratic Senate President Pro Tem John Campbell, of Windsor, and Republican Leader Joe Benning, of Caledonia, said Thursday evening they didn’t wish to comment on his arrest.

McAllister, 63, served four terms in the House in the last decade before election to the Senate.

He grew up in the southeastern Vermont town of Windsor, got a degree in agriculture management from Vermont Technical College and moved to Highgate, in Vermont’s far northwest corner, while in his 20s.