WASHINGTON — President Donald Trump’s impeachment trial is shifting to questions from senators, a pivotal juncture as Republicans lack the votes to block witnesses and face a potential setback in their hope of ending the trial with a quick acquittal. After Trump’s defense team rested Tuesday with a plea to “end now,” Senate Majority Leader Mitch McConnell privately told senators he doesn’t yet have the votes to brush back Democratic demands for witnesses now that revelations from John Bolton, the former national security adviser, have roiled the trial. Bolton writes in a forthcoming book that Trump told him he wanted to withhold military aid from Ukraine until it helped with investigations into Democratic rival Joe Biden.