Drew DeSilver at Pew:
The vast majority of the regular and special Senate elections held since 2013 – 69 of 73 – have been won by candidates who belonged to the party that won that state’s most recent presidential race, according to a Pew Research Center analysis of election results going back to 1980. That represents a marked contrast with prior years: As recently as 2006, 12 of 33 Senate contests (36%) were won by candidates of different parties than their state’s 2004 presidential pick. (The election data we used came mostly from the Federal Election Commission, supplemented by information from the U.S. House Clerk’s office and media reports.)
