Facing a tough midterm election, Democrats put aside some of their remaining scruples about the new age of unlimited campaign spending and courted unions and hedge-fund billionaires for big checks to try to salvage a Senate majority. They spent a lot of money. But not enough. This year, in a reversal of 2012, the big-money Democratic donors watched their investments return little on election night Tuesday. Although Republicans outspent them overall, Democrats got beaten even in states like Colorado and North Carolina where they spent the same or even a little more than Republicans. "We just saw a national tsunami,"...