I have heard that the coattails effect is where a candidate gets elected because the president is popular in the area they want to represent. I feel like it is getting stronger over time. Ticket splitting appears to be rarer in Democratic voters:
For example, in 2012 and 2016, over 90 percent of districts voted for the same party for the president as the House. And, in the 2016 elections, the elected senator's party could always be predicted accurately by looking at the party the state voted for for President. House:
Has the coattails effect become a stronger force?