WASHINGTON — Senators voted Saturday to consider hearing from witnesses in the impeachment trial of Donald Trump, potentially extending proceedings that just hours earlier had seemed to be speeding to a swift conclusion and up-or-down vote over whether the former president incited the deadly Capitol siege. The last-minute fight over witnesses followed Friday night revelations from a Republican House lawmaker about a heated phone call on the day of the riot between Trump and House Minority Leader Kevin McCarthy that Democrats say establishes Trump’s indifference to the violence. The proceedings came to abrupt halt Saturday morning, with even senators seemingly confused about next steps.