Although not well-known nationally, he's well-known in the Midwest where the key battles are. He's evidently a pragmatist and a centrist, and far more appealing than the weirdo Vance.
I expect nearly all of Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin are in play for the Dems now. Even, or perhaps especially Ohio, where Vance is so unpopular.