WASHINGTON—House managers said President Trump was a threat to American democracy, while his defense team countered that Democrats were trying to undermine two elections, in their final arguments in his Senate impeachment trial on Monday.

With Democrats set to fall well short of the two-thirds threshold needed to remove Mr. Trump from office in the final vote later this week, they used their turns at the lectern to issue a warning to the Senate, focusing on what they saw as the danger presented by Mr. Trump remaining in office.

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