“It’s an extraordinary moment,” said Steve Schmidt, a strategist on George W. Bush’s and John McCain’s presidential campaigns who opposes Trump. “The party has become completely unmoored from things that it held as close to sacred until very recently, including a fidelity to the country’s security institutions.”

The GOP offensive has raised fundamental doubts among millions of Americans about the independence and integrity of federal law enforcement agencies, which have not been caught in a political maelstrom of this magnitude since the Watergate scandal almost five decades ago.