Sen. Arlen Specter (R-Pa.) abruptly cancelled his appearance before a Senate Commerce panel hearing on human cloning last week because his prospective 2004 primary opponent, Rep. Pat Toomey (R-Pa.), asked to testify as well.

But GOP aides said Specter decided at the last minute to show up and testify at last Wednesday’s hearing. The incident revealed the potentially awkward situation bubbling in the Pennsylvania Congressional delegation.

Toomey has not been shy about saying he is “seriously considering” a run against Specter, whom he views as too moderate for the state. Specter is backing a Senate bill that would permit scientists to clone human cells for therapeutic uses. Toomey — a staunch anti-abortion lawmaker — believes all human cloning should be banned.

Aides said Specter’s original request to testify at the hearing prompted Toomey to seek equal time for his point of view.

Toomey denied his testimony was designed to clearly and publicly distinguish his views on abortion and cloning from Specter’s in advance of a potential primary run.

“I weighed in on this because I feel strongly about it,” said Toomey. “I doubt that a lot of people will base their voting decision on this issue.”

Still, Pennsylvania political observers have noted that Toomey’s best chance at besting Specter in a primary for the four-term lawmaker’s Senate seat is in energizing religious conservatives who have been disappointed with Specter’s defense of abortion rights. Toomey has been mulling a Senate run because he promised to serve only six years in the House upon first being elected in 1998.

The Philadelphia Inquirer reported last month that White House aide Karl Rove discouraged Toomey from challenging Specter in a state that President Bush lost narrowly in 2000. Nevertheless, some conservatives are still urging Toomey to launch a campaign.

Specter’s obvious discomfort at being seated at the same witness table with Toomey manifested itself in his abbreviated statement and his prompt retreat from the hearing room.

Though Specter left, Toomey and Rep. Dave Weldon (R-Fla.) — the House’s lead sponsor of a bill to ban human cloning — stayed for almost an hour defending their position that all human cloning, regardless of its usefulness in curing diseases, should be outlawed.

Specter said his on-again, off-again testimony was unrelated to Toomey’s request. “I wanted to testify if I could get the schedule worked out,” said Specter, explaining that he had originally planned to chair a Veterans’ Affairs Committee hearing at the same time as the Commerce hearing. That was postponed.

Sen. Sam Brownback (R-Kan.), who presided over the cloning hearing and is the leading Senate sponsor of a complete ban on human cloning, said he had not anticipated the intra-party fight.

Specter is a co-sponsor of legislation that would allow scientists to clone human cells for research purposes but ban the creation of a cloned human child.

“It was our hope at the outset not to have Members testify,” said Brownback, though several Members requested to. “What we try to do is accommodate all Member requests.”