President Donald Trump is using the phrase "witch hunt" to describe the special counsel's Russia probe more frequently, as the investigation grinds on and Trump's former campaign boss goes on trial.

The most recent came Wednesday, as the president the fired off multiple tweets targeting special counsel Robert Mueller's investigation into Russian election hacking and potential links between the Kremlin and the Trump campaign.

"This is a terrible situation and Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now, before it continues to stain our country any further," the president wrote.

He used the phrase in a second tweet Wednesday.

Since Trump's inauguration, "witch hunt" has become the president's preferred description for investigations into Russian interference in the 2016 election. In May 2017, Trump declared the Russia probe "the single greatest single witch hunt of a politician in American history."

But over the past two months, as Mueller's probe marked its first anniversary and the trial of former campaign manager Paul Manafort approached, the frequency of Trump's "witch hunt" complaints have picked up sharply. He mentioned the phrase as many as nine times a week in late June.