One of the simplest explanations for Republicans majorities of nearly every level of government is President Barack Obama. Voters like a check on the party in the White House, and the check they put on Obama was particularly strong. So it stands to reason that the reverse is likely true: A President Donald Trump should be a boon for Democrats down ballot. And if the traditional rules of politics hold, it likely will be. But for Democrats, it’s not as simple as putting a Republican in the White House and hoping for the best.