Pennsylvania state Sen. Doug Mastriano, a fervent election denier who lost last year's gubernatorial race, said he's not running for US Senate on Thursday night, a relief for some Republicans who had feared the controversial conservative would complicate their hopes of flipping a Democratic-held seat.

"At this time, we have decided not to run for the US Senate but to continue to serve in Harrisburg," he said on Facebook Live.

Mastriano, who had lost the governor's race by 15 points six months ago, had come under pressure to stay out of the race from some Pennsylvania and national Republicans, who were concerned he'd be too extreme to win a general election against Democratic Sen. Bob Casey, who's running for a fourth term.

This is a breaking story and will be updated.