Republicans are worried that Donald Trump, losing badly to Hillary Clinton, is dragging down the party’s candidates for Congress and state legislatures with him. Sens. Kelly Ayotte in New Hampshire, Richard Burr in North Carolina and Pat Toomey in Pennsylvania — once considered favorites for re-election — are now in tight races or losing. But presidential and Senate races are fought on the same terrain: state by state. Elections for the U.S. House of Representatives and state legislatures are more localized. So, how vulnerable are the GOP’s majorities in Washington and state capitols if Trump’s numbers stay where they are or worsen?
This question boils down to how closely tied presidential voting is with votes for Congress and state legislatures. There’s a pretty clear relationship — for each additional percent of the vote a presidential candidate receives, his or her party will gain several House seats and about two dozen state legislative seats, according to my analysis. But there’s quite a bit of leniency in that relationship. While having an unpopular candidate at the top of the ticket is certainly a challenge, it’s not necessarily a death sentence. Candidates can sometimes successfully distance themselves from their presidential candidate.
The table below gives us some indication of the relationship between presidential election results and party fortunes in Congress and state legislatures. For every presidential election since 1952, it shows the share of the two-party popular vote received by the incumbent party (the party in charge of the White House at the time of the election). It also shows the increase in the number of state legislative seats and the increase in U.S. House seats that party gained in that election:
Sources: U.S. House Clerk’s Office, National Conference of State Legislatures
There are a few things to notice here. For …