Senators will take an oath Tuesday to ensure "impartial justice" as jurors in Donald Trump's historic impeachment trial, proceedings that will test Republican loyalty to the former president and the power he still wields after the deadly siege at the U.S. Capitol. But some Republican senators are challenging the legitimacy of the trial and questioning whether Trump's repeated demands to overturn Joe Biden's election really constitute "incitement of insurrection" in the Jan. 6 riot.

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