WESTCHESTER COUNTY Republicans gathered at the County Center in White Plains with placards, bunting and political-convention music last week to designate candidates for Congress and the state legislature and to hear from the leaders of the statewide Republican ticket.
The nearly 200 delegates named 10 incumbents and selected familiar figures for two races. Assemblyman Nicholas A. Spano was named the candidate for the 35th District seat being vacated by State Senator John E. Flynn, Republican of Yonkers, and John M. Perone, a former Assemblyman, was chosen to challenge the Democrat Ronald C. Tocci of New Rochelle in the 85th Assembly District.
The Yonkers race has taken on statewide significance because Mr. Flynn's decision to retire at the end of this, his 10th term, could cause a problem for Republicans seeking to maintain a majority in Albany's upper house. With Republicans holding 35 of the 61 seats in the Senate chamber, a net loss of five seats would mean Democratic control.
Delegates at the two-hour convention heard from Andrew P. O'Rourke, the County Executive who was named a week earlier by state Republican delegates as their candidate for governor, and from District Attorney E. Michael Kavanaugh of Ulster County, who replaced Jeanine F. Pirro, an assistant district attorney for Westchester, as the candidate for lieutenant governor.
Mrs. Pirro withdrew her name from contention, according to Mr. O'Rourke, because she felt it would be unfair to clients of her husband's law firm to expose them to public scrutiny. She has since returned to work although Democrats in the county legislature are asking for an inquiry into possible conflicts of interest in her job as a prosecutor.
None of this emerged from the podium in the County Center, as the gubernatorial candidate spoke of the O'Rourke-Kavanaugh team, or ''an O.K. ticket,'' as he put it, and the issues they would raise. Mr. O'Rourke said Governor Cuomo was soft on crime and public welfare, for example, and was misleading when it came to assessing the state's economy.
Mr. Kavanaugh spoke in a similar vein, adding that corruption was an issue. ''Why has the Governor allowed the government of New York City to become hoplessly corrupt,'' he asked. They were cheered by the partisan audience, which then got down to the business of selecting candidates for local races.
They picked some newcomers including one, Willis H. Stephens Jr., with a familiar political name. His father served as an Assemblyman from Putnam County from 1953 to 1982 as did his grandfather, D. Mallory Stephens, from 1925 to 1952.
Mr. Stephens, 30 years old of Port Chester, will seek the 36th District Senate seat held by Suzi Oppenheimer, Democratic Mayor of Mamaroneck. A lawyer with a practice in Manhattan, Mr. Stephens also serves as assistant counsel to the Republican majority in the State Senate.
The delegates, following a 15-page convention script designed by Anthony J. Colavita, who is both the Westchester County and State Republican Chairman, selected Joseph Rauso, a 52-year-old engineer for the county's Department of Environmental Facilities to seek the 83d District seat being vacated by Mr. Spano.
Mr. Rauso, a Yonkers resident who has worked for the county for 30 years, said he lost by about 100 votes to Frank McGovern last November in the race for Yonkers City Council. Mr. McGovern, a Democrat, also had the Conservative Party designation for that race.
Robin Bergstrom, communications director for State Senator Mary B. Goodhue, a Republican seeking her fifth term, was designated by the nearly 200 delegates to challenge Assemblyman Richard L. Brodsky for the 86th District seat. The 39-year-old resident of Hastings-on-Hudson is a former radio and cable television news director.
Another newcomer, Joseph G. Goubeaud Jr. of Mount Vernon, was selected as the Republican candidate for the Assembly seat held by the Democrat Gregory P. Young, who won his last election by a 2-to-1 margin. Mr. Goubeaud, 36, a local lawyer entering his first political race, said he would be campaigning in a district with a 3-to-1 Democratic enrollment.
The delegates routinely designated three incumbent Republican United States Representatives; Joseph J. DioGuardi of New Rochelle for the 20th Congressional District, Hamilton Fish Jr. of Millbrook, N.Y., for the 21st District and Benjamin A. Gilman of Middletown for the 22d District.
The delegates also nominated the incumbent Assemblymen Gordon W. Burrows of Yonkers, Peter M. Sullivan of White Plains, Henry W. Barnett of Yorktown Heights, Vincent L. Leibell 3d of Carmel - whose district includes part of northern Westchester - and George E. Pataki of Peekskill.
And they backed two incumbent State Senators, Mrs. Goodhue, whose 37th District includes all of northern Westchester, and Guy V. Velella, a Bronx Republican who recently won election to the 34th District seat long held by the late John D. Calandra. The Velella seat includes parts of Yonkers, Mount Vernon and New Rochlle.
The delegates seemed to display great interest in the Spano designation for Mr. Flynn's seat. Frank Coppola, the Yonkers Republican chairman, said that ''John Flynn was a legend in his own time,'' but that ''time and a desire to be with his wife and family caused him to retire.'' But ''if anyone can keep his seat, it is Nicholas Spano,'' he said.
The candidate, one of 16 children of Leonard N. Spano, a county legislator from Yonkers, said all his brothers and sisters were present, as were other members of the family ''including a grandmother,'' and that together, with others, they would ''keep this seat Republican.'' The crowd loved it.
Mr. Perone, a former three-term Assemblyman, told the political gathering after his nomination that he was ''no newcomer'' to the political fray and had been out of office after losing a close race in 1984 to Mrs. Oppenheimer for the State Senate seat vacated by Joseph R. Pisani, the New Rochelle Republican.
Mr. Perone will be challenging Mr. Tocci for the Assembly seat that the Republican formerly held. Mr. Tocci, who will be running for his second term, was himself involved in a close contest in the 1984 election, narrowly beating a Conservative, Nicholas X. Judge 3d, who was also running on the Republican line.