Welcome to The Tip Sheet, a daily political analysis of the 2018 elections, based on interviews with Republican and Democratic officials, pollsters, strategists and voters.

Republicans on defense in South Carolina

Want to know which party is playing defense a week before the election? Take a look at where the independent expenditure arm of the National Republican Congressional Committee is going on the air at the last moment in its bid to protect the G.O.P.’s 23-seat House majority: South Carolina’s First District.

That would be the Charleston-area seat that is currently represented by Representative Mark Sanford and that has long been a bulwark of conservative strength. But Republicans are nervous that Katie Arrington, who defeated Mr. Sanford in the Republican primary after President Trump torpedoed him in the final hours, is proving to be a lackluster candidate. They don’t want to take any chances against a Democrat, Joe Cunningham, who has out-raised her.