WASHINGTON -- A special election to replace the late Sen. John Heinz of Pennsylvania is shaping up as this year's key political test, even though the identity of the candidates remains a mystery.
Among those prominently mentioned as potential candidates are Attorney General Richard L. Thornburgh, who is a former Pennsylvania governor, and two influential House Democrats, Representatives William H. Gray III of Philadelphia, the majority whip, and John P. Murtha of western Pennsylvania.
Besides altering the balance of power in the Senate, the race could provide clues to the long-term political impact, if any, of the Persian Gulf war.
"This will be the most watched Senate race in history," one Democratic insider predicted yesterday. The only other statewide elections scheduled this fall are governor's races in Louisiana, Kentucky and Mississippi.
A top Republican official called the statewide race "drop-dead" important. The GOP has hopes of retaking the Senate in 1992, but the party could find itself down 14 Senate seats if a Democrat were to win in Pennsylvania this fall.
"The Democrats have a realistic chance to capture this seat, which they really didn't have with John Heinz occupying it," said George Burger, a Democratic campaign consultant.
Mr. Heinz, a popular, three-term incumbent, was killed in a plane crash last week near Philadelphia and is to be buried today in his hometown of Pittsburgh.
Under state law, a special election will be held Nov. 5 to fill the remainder of Mr. Heinz's term, which expires January 1995. The Democratic and Republican state committees are to convene later this spring to choose nominees. The unusual off-year election could give elected officials in either party a free shot at the Senate, since they could return to their current post if defeated.
Gov. Robert P. Casey, a Democrat, has the option of appointing a temporary successor, or "caretaker," to serve on an interim basis until November. State Republican Chairwoman Ann Anstine has urged the governor to name Mr. Heinz's widow, Teresa, to the job, which would keep the seat in GOP hands at least through November. A number of prominent Democrats also have been suggested as caretakers.
Mr. Casey could decide, however, to appoint as interim senator a fellow Democrat who would then use the position as a springboard for the November race. The governor has resolutely refused to discuss the matter until after Mr. Heinz's funeral, and those who know him well say it is virtually impossible to predict what he will do.
Mr. Thornburgh, who left the state in 1987 to become attorney general in the Reagan administration, has yet to say publicly whether he is interested. But most analysts see him as the GOP's strongest candidate and the early favorite if he decides to run.
After Mr. Thornburgh, the Republicans are left with a variety of possibilities who lack strong statewide identification, including Representative Tom Ridge, state Attorney General Ernie Preate Jr., state Representative Stephen Freind, an anti-abortion leader, and former U.S. Transportation Secretary Drew Lewis.
Among Democrats, Mr. Gray has indicated recently, through aides, that he is looking at the Senate contest. Neil Oxman, a Democratic consultant in Philadelphia, said Mr. Gray could win a statewide contest, even though his own supporters admit that the congressman, who is black, would face a significant hurdle because of his race.
Another possibility, Mr. Murtha, a moderate conservative, is politically close to the governor. He has told reporters from his district that he might be interested in taking a close look at the race, his spokesman said.
Other potential Democratic candidates include Lt. Gov. Mark Singel, state House Speaker Robert O'Donnell and state Treasurer Catherine Baker Knoll.