Trump is tweeting 'witch hunt' a lot more than he used to, as Mueller probe grinds on and Manafort goes on trial 

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.

Manafort worked for pro-Russian politicians in Ukraine. While prosecutors have said they will not present evidence in his trial about possible collusion between Russia and the Trump campaign, they may dig deeper into Manafort's connections with Russian and Ukrainian oligarchs, legal experts have said.

Wednesday's tweet wasn't the first time Trump has attacked his attorney general for recusing himself from the Russia probe in March 2017. Sessions cited his role as a senior advisor to Trump's presidential campaign and appointed Deputy Attorney General Rod Rosenstein to oversee the investigation. Rosenstein, in turn, appointed Mueller.

U.S. intelligence agencies concluded last year that Moscow interfered with the 2016 presidential campaign to try to tip the vote in Trump's favor. Moscow has denied such interference, and Trump has denied any collusion by his campaign or any obstruction of justice.

Trump's tweets may themselves become part of Mueller's investigation into whether Trump or anyone in the campaign tried to obstruct justice. The New York Times reported last week that Mueller's team was examining negative tweets and statements by Trump about Sessions and former FBI Director James Comey.

A favorite phrase, even before the Mueller probe

The president's "witch hunt" ire hasn't always been directed at the Russia probe.

Trump first adopted the use of the phrase on Twitter well before Mueller's appointment in May 2017. In November 2011, Trump tweeted a link to a Fox News interview in which he defended then-GOP presidential candidate Herman Cain as the target of a media "witch hunt." A month later, Cain suspended his campaign following allegations of sexual harassment, which he denied.

On June 24, 2013, Trump referenced media coverage of New York Attorney General Eric Schneiderman's investigation into several New York banks, which Trump described as "a witch hunt against Republicans."

Trump deployed the phrase on Twitter again two months later, this time to refer to a "Liberal Witch Hunt" against Trump University. The Tweet came a day after Schneiderman sued Trump for $40 million, alleging that many of the 5,000 students paid as much as $35,000 to attend workshops that failed to provide promised access to real estate experts.

Trump denied the claim. His attorney accused Schneiderman of trying to extort campaign contributions. The suit was later settled for $25 million. Schneiderman resigned in May following allegations of sexual and physical abuse by three former romantic partners. In a statement, he "'strongly contested'" the allegations, but resigned because they would "'effectively prevent'" him from doing.

— Reuters contributed to this story.

WATCH: Trump fires back at his former personal lawyer Michael Cohen

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