This article is by Katharine Q. Seelye, Jim Rutenberg and Jeff Zeleny.

Shortly after Representative Joe Sestak won an improbable victory Tuesday over Senator Arlen Specter in Pennsylvania’s Democratic Senate primary, President Obama called Mr. Sestak to congratulate him. The president pledged his full support, aides said later, and offered to campaign for him in the fall — if Mr. Sestak believes it will help.

Nothing makes friends like winning. It was not long ago, Mr. Sestak said Wednesday in an interview, that White House officials were so eager to muscle him from the race that they offered him a job if he would drop out.

Mr. Sestak remains mum on the details, except to say that it was a high-ranking post — secretary of the Navy has been mentioned as a possibility — and that it happened last summer. The White House, which had backed Mr. Specter, has denied the assertion.

What is clear is that Mr. Sestak, 58, a former Navy admiral with a reputation as a hard-charging and demanding taskmaster, has no hesitation about defying the White House or other powerful interests.