Castro will be an active surrogate for Warren who is planning to travel the country for her, according to people familiar with the plans. He will keep a handful of staffers from his campaign with him while others from his policy team are likely to join hers.
The endorsement forms a political marriage of like-minded campaigns and candidates. Castro and Warren are credited by the rest of the field for churning out some of the most detailed and liberal policy proposals, on issues like immigration, criminal justice reform and taxes. Castro often released his plans first and Warren followed with her own similar proposals, which inevitably garnered more attention.
Not ol' Joe. Not young Pete. Nor Amy. Castro knows quality when he sees it; and knows Biden after years of Biden's love of credit card companies and such.
Is it a surprise, with Warren not blessed with offspring like Hunter Biden? Without all those Senate years of being Joe?
Last, I do not know Corey Day, but am I supposed to be impressed with his judgment? Likely there's a salary involved. A breakaway from Minnesota Dem gravitation in large measure to embracing the Klobuchar candidate; with no signs of Day having been offered a job with Amy.
Go where the getting is good?
_____________UPDATE_____________
The Castro endorsement is more than a formal one, appearing to involve staff adjustments and support in campaigning much as AOC has done with Bernie - her endorsement being more than merely formal, although she has to concentrate on reelection this year. Castro has no such other opportunity/duty, at present. See, e.g., NYTimes. Newsweek.