Charles S. Robb, a former governor and son-in-law of President Lyndon Johnson, defeated a longshot GOP opponent Tuesday to become Virginia's first Democrat elected to the Senate since 1966. Republican George Bush overwhelmed Democrat Michael Dukakis for the state's 12 electoral votes. With 14 percent of the precincts counted, Bush had 166,202 votes or 58 percent and Dukakis had 118,962 votes or 42 percent. With 14 percent of the precincts counted, Robb had 203,493 votes or 73 percent, and Republican Maurice Dawkins had 76,874 votes or 27 percent. Robb and Dawkins, a retired black minister and Washington lobbyist, competed for the seat of Republican Paul S. Trible Jr., who passed up a bid for a second Senate term and plans to run for governor in 1989. Robb outspent Dawkins by about 10 to one. Also on the ballot was a pari-mutuel referendum. Virginia voters narrowly defeated horse-track wagering 10 years ago, but a lottery referendum passed easily last year. Five Democrats and five Republicans were expected to win re-election to the U.S. House.