AMERICANS today will elect a senator in Pennsylvania in a race testing President Bush's popularity, choose governors in Mississippi and Kentucky, and vote on right-to-die, tax, and term-limit initiatives in Washington state.Those are highlights of the 1991 elections, in which only one senator, two congressmen, two governors, four state legislatures, and about 40 mayors will be chosen. The national focus is on Pennsylvania, where voters will select a successor to the late Sen. John Heinz. Former United States attorney general Dick Thornburgh, who was also a two-term governor of the industrial state, is challenging Democratic Sen. Harris Wofford, who was appointed to succeed Senator Heinz. A Wofford win would be a stunning upset, pundits say, and would be interpreted as embarrassing to President Bush. In the gubernatorial races today, Democratically controlled statehouses in Mississippi and Kentucky, once believed ripe for the plucking by Republicans, now appear out of reach for Bush's party. The Democratic candidates are relative liberals in conservative states that Republicans had high hopes of capturing. But polls taken last week showed the Democrats had solid leads. In the Mississippi contest, Democratic incumbent Ray Mabus, a Harvard-educated reformer, had about a 14-percent lead against Republican millionaire businessman Kirk Fordice. Governor Mabus is a solid favorite to become the state's first chief executive elected to consecutive four-year terms. In Kentucky, Democratic Lt. Gov. Brereton Jones, a millionaire horse breeder, had a 10-point lead ahead of Republican Rep. Larry Hopkins. Democratic Gov. Wallace Wilkinson was barred from seeking reelection. In various issues facing voters, a Washington state referendum on term limits threatens to wipe out all of the state's congressional delegation by 1994. Unlike term limits adopted last year in Colorado, California, and Oklahoma, the Washington State referendum would count the years already served toward a six-to-12 year ceiling, depending on the office. That would bar half the the state's US House delegation from seeking reelection in November 1992 - including Speaker Thomas Foley (D), the third-ranking public official in the United States. Washington voters will also decide on other issues including abortion, whether doctors should be allowed to help their terminally ill patients commit suicide, and new taxes.