CLEARWATER, Fla. — For as long as most people here can remember, Representative C. W. Bill Young presided over this slice of Florida’s west coast, tending assiduously to veterans and retirees, crystalline beaches, and a booming population. By the time he died at 82 two months ago after 43 years in Congress, Mr. Young was the longest-serving Republican in Congress, and with good reason: He was untouchable at the ballot box.

Mr. Young’s personal dominance kept the seat frozen in place for four decades. But beneath the surface, changes in the area’s electoral map including an influx of Democrats — some of them gay voters and young people — have diluted the share of moderate Republicans, turning it into one of the rare commodities in American politics: a true swing district. Now, his absence has set off a contest in the first race of the 2014 battle for control of Congress, with both parties hoping for a victory and watching carefully how President Obama’s health care law may affect the outcome.

Determined to snatch the long-awaited open seat in the March 11 special election, Democrats effectively cleared the field for Alex Sink, a former chief financial officer of Florida, who ran for governor and lost in 2010. Ms. Sink did not even live in the district, Pinellas County, in October; she packed up and moved one county over last month.

Image C. W. Bill Young died in October. Credit... Cliff Owen/Associated Press

Shortly after, three little-known Republican candidates jumped into the fray. They find themselves competing not only against one another in the Jan. 14 primary but against the hustle and bustle of the Christmas season, in which bah-humbug advertising can spoil the merriment. A Libertarian candidate, Lucas Overby, also joined the contest.