WASHINGTON — A loose association of current and former Republicans working to sink President Donald Trump’s reelection now see another political feat that’s necessary for them to have a shot at reclaiming their party: flipping the U.S. Senate to the Democrats. The emerging belief, based on more than a dozen interviews, is that defeating Trump alone is insufficient to spur the reckoning required to salvage a party that will almost undoubtedly confront a crossroads if the president loses to Joe Biden this fall.