A Joe Biden victory on Nov. 3 would have a sweeping impact on mergers and acquisitions as he sets his sights on a higher corporate tax rate, new regulation and increased antitrust scrutiny. The biggest loser might be private equity dealmakers. If firms don’t offload assets before the end of the year, they could have to pay almost double the current rate of capital gains tax on any profits -- a hit that may also dissuade founder-backed companies from pursuing a sale. But it could be a busy...