He’s been running for this slot for awhile, it would seem. I find this in a Jan 2003 Jim Geraghty NRO piece:
In his Hardball appearance, Biden also made some oddly complimentary comments about two of his possible primary opponents, Massachusetts Senator John Kerry and North Carolina Senator John Edwards.
“I think Kerry is the strongest candidate in the field,” Biden said. “[Edwards] has a four-foot vertical jump, you know, and you know he may be able to step up to this … All kidding aside, he’s the single best natural candidate I’ve ever seen.”