Former US president Donald Trump's defense team Monday urged the Senate to dismiss his impeachment trial on the grounds it is constitutionally flawed, and warned Democrats were conducting "political theater" instead of fair justice.

"The Article of Impeachment presented by the House is unconstitutional for a variety of reasons, any of which alone would be grounds for immediate dismissal," Trump's lawyers said in a 78-page brief on the eve of his trial on charges of inciting a deadly riot at the US Capitol.

"They demonstrate conclusively that indulging House Democrats hunger for this political theater is a danger to our Republic democracy and the rights that we hold dear."

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