A little more than three months ago, as Democrats cast their ballots in the Nevada caucuses, Republicans felt confident about their chances in 2020. The coronavirus seemed a distant, far-off threat. Democrats appeared poised to nominate a self-described socialist for president. The stock market was near a record high. The economy was roaring. President Donald Trump looked well-positioned to win a second term, and perhaps pull enough incumbent Republicans along with him to hold the party’s majority in the Senate.