Which isn't much, congress controls the money and military aid given. Outside of joining the war and placing our military where IDF is and seeing if that will end war crimes, there's not a lot on the table and Bibi isn't going to take a single threat seriously, he knows the US is chained to Isreal with millions and millions of dollars in lobbying.
Can't remember at the moment what it's called, but there's a law making it illegal for the US to support war crimes. If the president ceased aid to Israel on those grounds, congress would have to be unusually united to override it. Not saying it couldn't happen, but it'd be a difficult fight to pick when Israel is being so brazenly genocidal.