The transportation secretary, one of several ambitious Democrats jockeying for prominence, gave advice in a speech but was coy about his own plans: “I know that I will make myself useful again later.”Pete Buttigieg outside the White House in July. On Thursday, he warned in a speech that Democrats “will be inclined to react with shock by some things that are done precisely with the intent of shocking us,” but that they needed to “move very quickly through the shock.”