Rachel Maddow's ratings took a 19 percent nosedive following the announcement that the Robert Mueller probe turned up no evidence of collusion between the Trump campaign and Russia.

The MSNBC host, whose show has proved essential viewing for those convinced of Kremlin interference in the 2016 election, drew a reduced 2.5 million audience on Monday, the day after Attorney General William Barr's announcement.

Having lost almost one fifth of her ratings overnight, her audience dwindled further Tuesday, plummeting to 2.3 million, data and measurement company Nielsen said.

Maddow has been one of the loudest anti-Trump voices, regularly dedicating extended segments of her self-titled 9pm show to detail alleged Russian interference and manipulation of President Trump's administration.

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Rachel Maddow's ratings took a 19 percent nosedive following the announcement that the Robert Mueller probe revealed no evidence of collusion between the Trump campaign and Russia

Maddow has been one of the loudest anti-Trump voices, regularly dedicating extended segments of her self-titled 9pm show to detail alleged Russian interference and manipulation of President Trump's administration

She saw her ratings surge from an also-ran to a top contender thanks to her compelling and dogged reporting on Trump - regularly landing the No.1 spot in her time slot slugfest with Fox News' Sean Hannity.

Her show built to a crescendo in the days leading up to the release, as rumors swirled that Mueller was nearing the conclusion of the report which Maddow confidently predicted would prove decisive in the downfall of Trump.

On Friday she ended a trout fishing trip early to broadcast an emergency episode from a studio on the University of Tennesse campus.

She said as the show began: 'A couple of hours ago - maybe even less than that - I was standing knee-deep in a trout stream in Tennessee.

'But now it's Mueller time, and so I'm in a studio in the great state of Tennessee.'

She saw her ratings surge thanks to her compelling and dogged reporting on Trump - regularly landing the No.1 spot in her time slot in a slugfest with Fox News' Sean Hannity

Her show built to a crescendo in the days leading up to the release, as rumors swirled that Mueller was nearing the conclusion of the report which Maddow confidently predicted would prove decisive in the downfall of Trump

However, after Barr's four-page summary on Sunday appeared to clear Trump of any electoral wrongdoing, the anti-climax appears to have put off some viewers, though whether this is a long-term trend or just a hangover remains to be seen.

Nonetheless, Maddow used her shows this week to call for the release of the full investigation suggesting the summary may not totally have exonerated Trump.

In the four-page summary, Mueller found that there was conclusive evidence that Russia did not interfere in the election, both through a coordinated campaign of disinformation and by hacking emails from Hillary Clinton's election team.

In a letter to lawmakers, Barr said that Mueller found that there had been 'multiple offers from Russian-affiliated individuals to assist the Trump campaign.'

But quoting directly from Mueller's report, Barr said that the special counsel's investigation 'did not establish that members of the Trump campaign conspired or coordinated with the Russian government in its election interference activities.'

However, after Barr's four-page summary on Sunday appeared to clear Trump of any electoral wrongdoing, the anti-climax appears to have put off some viewers, though whether this is a long-term trend or just a hangover remains to be seen

In the four-page summary, Mueller found that there was conclusive evidence that Russia did interfere in the election, both through a coordinated campaign of disinformation and by hacking emails from Hillary Clinton's election team

Many observers had predicted the biggest danger to Trump came from a possible accusation of obstruction of justice, particularly over his decision to sack the FBI director James Comey, who headed the investigation before Mueller.

But Barr said that the evidence outlined in Mueller's report 'is not sufficient to establish that the President committed an obstruction-of-justice offence.'

'In cataloging the President's actions, many of which took place in public view, the report identifies no actions that, in our judgement, constitute obstructive conduct,' Barr added in his letter.

But while Barr, who was appointed by Trump, concluded that the president had not obstructed justice, he acknowledged that Mueller himself was inconclusive on the question of obstruction.

'The Special Counsel therefore did not draw a conclusion, one way or another, as to whether the examined conduct constituted obstruction,' he said.

'The Special Counsel states that 'while this report does not conclude that the president committed a crime, it also does not exonerate him.'