Far-right influencers have been hinting in recent weeks that they have finally found a venue — Miami — and a federal prosecutor — Jason A. Reding Quiñones — to pursue long-promised charges of a “grand conspiracy” against President Trump’s adversaries.Their theory of the case, still unsupported by the evidence: A cabal of Democrats and “deep-state” operatives, possibly led by former President Barack Obama, has worked to destroy Mr. Trump in a yearslong plot spanning the inquiry into his 2016 campaign to the charges he faced after leaving office.But that narrative, which has been promoted in general terms by Mr. Trump...