WASHINGTON It became a regular litany of grievances from President Donald J. Trump and his supporters: The investigation into his 2016 campaign's ties to Russia was a witch hunt, they maintained, that had been opened without any solid basis, went on too long and found no proof of collusion.Egged on by Mr. Trump, Attorney General William P. Barr set out in 2019 to dig into their shared theory that the Russia investigation likely stemmed from a conspiracy by intelligence or law enforcement agencies.