Just a month after national Republicans appeared to give up on Arizona Republican Blake Masters, the Trump-backed Senate candidate has staged a comeback — and Arizona is in play. National and Arizona Democratic operatives are privately fretting that incumbent Sen. Mark Kelly is endangered, noting a shift by previously undecided voters toward the GOP, aided by a patchwork of new Republican spending to bolster Masters. Recent polling has shown Kelly’s lead shrinking to a few points, much tighter than surveys showed throughout August and September. “There’s a very narrow path to victory for Democrats in Arizona,” said Roy Herrera, a...