President Trump took to Twitter on Thursday morning to rail against the investigation into allegations of Russian collusion with his campaign, hours after a special counsel was named to oversee the probe.

Trump deployed his characteristic social media bombast to take his unvarnished response directly to voters, elaborating on the staid written statement issued the night before that denied any collusion with Kremlin-linked officials.

“With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special counsel appointed!” Trump tweeted. “This is the single greatest witch hunt of a politician in American history!”

Trump fired back ahead of a briefing Thursday afternoon for all U.S. senators being held by Deputy Attorney General Rod Rosenstein. The senior DOJ official, who made the call to appoint a special counsel, is set to brief lawmakers on Trump's controversial decision last week to fire FBI Director James Comey, a move that escalated calls on Capitol Hill for an outside counsel.

The pair of tweets on Thursday echoed advice Trump gave Wednesday afternoon to graduates at the U.S. Coast Guard Academy, in which he urged the cadets never to give in and always to fight back.

“Over the course of your life you will find that things aren’t always fair, not always warranted but you have to put your head down and fight, fight, fight,” Trump said. “Never ever give up. Things will work out just fine.”

In the speech, he also decried his treatment at the hands of the media, saying “no politician in history has been treated worse or more unfairly.”

The Twitter messages and Coast Guard Academy remarks were in stark contrast to the statement issued by Trump after the Wednesday night appointment of former FBI Director Robert Mueller as special counsel to investigate alleged Russian meddling in the U.S. presidential election and other matters.

“As I have stated many times, a thorough investigation will confirm what we already know – there was no collusion between my campaign and any foreign entity,” Trump said. “I look forward to this matter concluding quickly. In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country.”

A senior administration official told Fox News that Trump was measured in his response to news of Mueller's appointment on Wednesday, telling his staff it was time to move forward. Some members of Trump's team were surprised the president's reaction wasn't more visceral -- though Trump eventually ramped up his response in public with the Thursday tweets.

Fox News' John Roberts contributed to this report.