At age 43, U.S. Sen. Cory Gardner of Colorado is the youngest lawmaker in a generation to lead the national fundraising machine dedicated to expanding Republican ranks in the Senate. Not so long ago, it was the kind of job an ambitious politician could use as a steppingstone to something more — and, in Gardner’s case, it still could. But things are looking rough right now. The National Republican Senatorial Committee has seen a recent plunge in fundraising, driven by what many donors see as the party’s stymied agenda in Congress. Potentially more troublesome is the civil war between party leaders and Republicans aligned with former White House adviser Steve Bannon.