Democratic hopes for a wave election that would carry them to a significant House majority have been tempered in recent weeks amid a shifting political landscape and a torrent of hard-hitting attack ads from Republicans. Democrats remain favored to win, but GOP leaders believe they can minimize the number of seats they would lose — and, perhaps, find a path to preserving their advantage in the chamber. The tightening, with just over two weeks left, reflects how President Trump’s rising approval rating and the polarizing fight over Supreme Court Justice Brett M. Kavanaugh appear to be boosting the party’s candidates...