In a recent Politico article on "Swing-state Republicans warn Trump's reelection is on shaky ground," former NC Governor Pat McCrory observed "the concern is: Will he have coattails for the other offices, from Senate to governor and other important races?" Geoffrey Skelley at FiveThirtyEight.com also looked at the effect of presidential coattails on a variety of down-ballot contests through various studies by political scientists. He comes to the conclusion that while presidential coattails can certainly help down-ballot party candidates, the inverse may be true as well: the president could bring down those lower-ballot candidates as well.
For most political scientists and historians of American politics, the concept of presidential coattails is that the 'top of the ticket/ballot,' that is the president, would bring in lower-level candidates (U.S. Senate and House candidates, for example) who might not have been able to win on their own accord. In other words, presidents could potentially drag their party's candidates across the finish line to victories in the general election.
With the increase in partisan loyalty of American voters, one of the key questions asked about voter behavior is whether the top of the ticket is bringing in the votes for lower level office candidates of the same party, or could the top of the ticket represents voters who vote 'straight party' down the ballot?