Roy Moore says he will not accept defeat in Alabama’s election, arguing the vote was “tainted” by outside groups trying to stay in power.
In a four-minute video on YouTube, the hardline Republican lashed out after Democrat Doug Jones claimed victory in Tuesday’s election.
Mr Moore railed against gay rights, abortion, and “the right of a man to claim to be a woman and vice versa”.
Democrats will hold the Alabama senate seat for the first time in 25 years.
“We are indeed in a struggle to preserve our republic, our civilisation and our religion,” Mr Moore said in fire-and-brimstone remarks.
The election race, he claimed, was “tainted by over $50 million from outside groups who want to retain power and their corrupt ideology”.
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The Christian conservative ex-judge also said the American people “no longer recognise the universal truth that God is the author of our life and liberty”.
“Abortion, sodomy and materialism have taken the place of life, liberty and the pursuit of happiness,” he added, referring to a famous passage in the US Declaration of Independence.
Mr Moore crashed to defeat after he was engulfed by multiple allegations of sexual misconduct, including child molestation.
“In this race,” he said, “we have not received the final count to include military and provisional ballots.”
He added that he was awaiting final certification of the results.
Mr Jones won 49.9% of the vote compared to Mr Moore’s 48.4%, a margin of nearly 21,000 votes out of 1.3 million cast, US media report.
Alabama’s secretary of state has said overseas ballots can continue to come in until Tuesday next week.
But he said it was “highly unlikely” the Democrat will not be formally declared the winner.