WASHINGTON—House lawmakers prepared for a final round of debate Wednesday that was expected to culminate in votes to impeach President Trump over charges he abused the power of his office and obstructed Congress.

Potential drama over the outcome evaporated after Democratic fence-sitters largely lined up against Mr. Trump. Nearly all of the 31 Democrats who represent districts that Mr. Trump won in 2016 have said they would support impeachment, giving Democrats enough support to pass the articles without any Republican votes.

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