MADEIRA BEACH, Fla. — Pinellas County, with its off-the-clock retirees and gentle Gulf Coast beaches, cultivates a life of flip-flops and easy breathing.

But with just days to go before a special election that will kick off the high-stakes battle for the control of Congress, serenity is not easy to come by for anyone with a television, mailbox or phone. Both parties and their surrogates have swooped into this laid-back House district, which was overseen by Representative C. W. Bill Young, a Republican, for four decades until his death last year, and shaken it up like a dime-store sand globe.

Outside groups have dumped millions of dollars into mostly acrimonious messages meant to fire up voters and test the potency of the Affordable Care Act as an election issue. That snarling tone has rubbed off on the two major candidates, both relative moderates: Alex Sink, a Democrat and Florida’s former chief financial officer, and David Jolly, a Republican, former lobbyist and former aide to Mr. Young. They have traded accusations over a wide variety of issues, including immigration reform and Social Security.

The third candidate in the special election on Tuesday, Lucas Overby, a Libertarian, has mostly stayed out of the fray.