STAMFORD, Conn. (AP) _ Voters in Connecticut's 4th Congressional District today elected a successor to Republican Stewart McKinney, who died in May of complications from AIDS.

Analysts said hot weather, the vacation season and noon to 8 p.m. voting hours might curb interest in the contest between Democrat Christine Niedermeier, 35, a former state representative and a Fairfield attorney, and Republican state Rep. Christopher Shays, 41, a Stamford real estate agent.

''Perhaps more than any other election in recent memory, this one depends on the turnout,'' said Jonathan Pelto, the state Democratic Party's political director. ''If people come out and vote, Chris Niedermeier will win.''

Robert S. Poliner, chairman of the state Republican Party, said predictions of a 25 percent turnout by political analysts ''are disappointing and I hope wrong.''

Although Democrats outnumber Republicans in the district, it is considered the heart of the Connecticut GOP and the race is being watched closely in Washington. There are about 105,000 Democrats, 91,000 Republicans and 73,000 unaffiliated voters in the district.

At the east end of the coastline district is Bridgeport, Connecticut's largest city and one of the nation's poorest. At the west end is Greenwich, one of the richest communities in the country.

Steven Lotterer, a spokesman for the National Republican Congressional Committee, said the turnout ''is a little hard to predict because no one has reliable insight to what happens when polls open at noon in the middle of August.''

Also on the ballot is independent candidate Nicholas Tarzia, a Stamford plumber running on the War Against AIDS ticket.

McKinney, 56, died May 7 of AIDS-related complications after holding the seat for 17 years.