"Keep your powder dry" is the message two of Pennsylvania's most influential Republicans, Sen. John Heinz and former secretary of transportation Drew Lewis, gave to Pennsylvania GOP legislators last week, urging them to remain neutral for the time being in the presidential race.

Vice President Bush's organization has had early success rounding up support, gaining endorsements from five of the state's 13 GOP House members and nearly one-third of the GOP state representatives. But Lewis and Heinz want to put the brakes on. They argue that it is too early to commit and that remaining neutral improves the state's bargaining position. They also expressed concern about fallout from the Iran-contra hearings.

"It's nothing against Bush," said state party Chairman Earl Baker. He said Heinz, as the leader of the state delegation, wants to set the timing for any commitment and "won't rush into this." He added that the state's junior senator, Arlen Specter, will take his signal from Heinz. Baker did note, however, that Specter has boyhood ties to Sen. Robert J. Dole (Kan.) and that Lewis has been reported to favor Dole.

And there is another reason to resist, Heinz told the group: "We may have a candidate of our own." There are rumbles that former governor Richard L. Thornburgh is considering joining the race. "I would describe it as a very preliminary and exploratory kind of effort," a former Thornburgh aide told The Philadelphia Inquirer. "It is literally testing the waters."

The Inquirer reported yesterday that Thornburgh, director of the Institute of Politics at the John F. Kennedy School of Government at Harvard, and his supporters consider Bush a vulnerable front-runner. As one Thornburgh backer put it: "This thing could open up."