It’s being reported by multiple outlets, and the word reportedly comes from close associates:

Pat Toomey has begun telling friends that he will run for U.S. Senate against Republican Arlen Specter in 2010.

Two sources said they spoke with the former Lehigh Valley congressman this week and were told that he’s made up his mind about a repeat challenge of the five-term incumbent in next spring’s GOP primary. Toomey lost narrowly to Specter in the 2004 primary.

Richard Thulin, head of the 20-member Lehigh Valley Republican Network, sent an e-mail to others within the group today telling them that “Pat’s formal announcement will be forthcoming.”

“Interesting news,” he wrote. “Pat Toomey asked me to let you know that he has decided to challenge Arlen Specter…”

His decision to jump into the GOP primary, meanwhile, could alter the race on the other side of the aisle. Only Joseph Torsella, former president of the National Constitution Center, has committed to a run in the Democratic primary. But political observers say they expect many more to run if they think Specter is vulnerable from the right.

Among those considering runs are U.S. Reps. Allyson Schwartz, Patrick Murphy and Pennsylvania auditor general Jack Wagner.

If Arlen Specter faces a primary challenge from Pat Toomey, he has to turn right, doesn’t he? It may be too late for him to earn meaningful points with conservatives, but he has to try. Does that mean he moves from being silent on Card Check to officially opposing it? Such a betrayal would make him enemies in the unions, but would give Specter helpful headlines about how his change of position saved the secret ballot. That would fit his agenda quite well – weakening him in the general election, where he’s a favorite, but strengthening him in the primary – where he really needs help.

Go read Michael Barone, who thinks that Specter is in serious danger in a primary – particularly against a credible challenger like Toomey – but is also a favorite in a general election. Toomey might need a strong wind at his back to win – but hey, Rick Santorum parlayed a strong GOP year into 12 years in the Senate as one of its leading conservative voices. Maybe Toomey will wind up doing the same.