As Democrats try to hold the Senate through defending red states, a look at the rise and fall of split Senate outcomes in presidential years.KEY POINTS FROM THIS ARTICLE— In the post-World War II era, there have been nearly 200 instances of states voting for one party for president and another for Senate in presidential election years.— This type of ticket splitting has generally been to the benefit of Democrats, who have been out of the White House for a slight majority of that timespan.— The split-ticket trend has been declining, but Democrats will want to reverse that to some degree this year.