WASHINGTON — It has been called President Joe Biden’s “shadow army,” described as a strike force to shake down small businesses with assault rifles and likened to a militia of auditors on search-and-destroy missions. Decades of Republican antipathy toward the Internal Revenue Service have reached new levels of enmity with the passage of a Democratic-backed bill that gives the agency $80 billion to beef up its ability to go after tax cheats. But Republicans, who have long accused the IRS of unfairly targeting conservatives, have seized on the law to fan unfounded conspiracy theories about the threat that mom-and-pop shops...