March 19 (Bloomberg) — When President Barack Obama flew to Miami earlier this month, Air Force One steered clear of the St. Petersburg area, where Democrat Alex Sink was in the stretch run of a tight race for an open House seat.

The president, stuck below 50 percent in national approval-rating surveys, wasn't welcome in that part of the Sunshine State. The tale of two Floridas, where Obama returns tomorrow to raise money at the Miami-area home of former professional basketball star Alonzo Mourning, is instructive for Democratic candidates in tough midterm elections this year.

Raising money is the least — and the most — Obama can do for them. That's true for both swing-district House members and the four most vulnerable Senate Democrats on the ballot in November: Mary Landrieu of Louisiana, Mark Pryor of Arkansas, Kay Hagan of North Carolina and Mark Begich of Alaska, who all hail from states where Obama's aggregate approval rating in 2013 was 43 percent or below, according to Gallup.