CNN

Sestak is weighing a 2010 Senate bid.

(CNN) - Pennsylvania Rep. Joe Sestak told CNN Sunday that he wasn't sure Sen. Arlen Specter is really part of the Democratic Party, the latest in a series of tough comments aimed at his potential Senate primary rival.

"I'm not sure he's a Democrat yet," he told John King on State of the Union.

President Obama said earlier in the week that Specter had his support, with one administration official saying it took the president about "seven seconds" to make that decision.

Those remarks don't faze Sestak.

"The president has said he respects Arlen's independence..." he said. "He'll respect mine if that is the case, I know that."

After Specter's announcement last week, Sestak said he was taking a wait-and-see approach on the question of whether to embrace the party's newest senator. Later in the week, he finally confirmed publicly that he's weighing a Senate bid of his own, setting up the prospect of a primary fight between the Democratic congressman and the party's newest senator.

In an effort to give a boost to any potential Democratic primary challenger, Specter's former colleagues in the Senate GOP launched a new campaign last week designed to highlight the consistency of his Republican record and his ties to President Bush and other party leaders.