New York (CNN) Michael Cohen, Donald Trump 's longtime protector and former attorney, now wants to be the man to take down the President.

In a matter of months, Cohen has gone through an extraordinary transformation: from a self-proclaimed "fixer" eager to do whatever it takes to shield his boss, to an aggrieved Democrat strategizing ways to help bring about the President's political -- and legal -- downfall.

Cohen has all but flaunted his own metamorphosis.

Last week, Cohen's adviser Lanny Davis disclosed that the 52-year-old had changed his party registration back to Democrat. A source told CNN this week that Cohen is willing to hit the campaign trail and donor circuit to fight Trump, and publicly call out anything he considered to be lies from the President. And since pleading guilty to multiple criminal counts over the summer, Cohen has been in contact with a range of investigators, including from special counsel Robert Mueller's office -- no doubt an effort to try to receive a lighter prison sentence.

In a rare tweet over the weekend, Cohen sent out a public plea: "The #MidtermElections2018might be the most important vote in our lifetime," he wrote, adding: "#GetOutAndVote#VoteNovember6th"

Cohen's message has been unmistakable: He is now prepared to do most anything to fight Trump.

Trump has returned the hostility -- in much blunter terms.

In an interview with the Associated Press on Tuesday, the President derided Cohen as a "low level" PR person who did "very small legal work" for him. Trump also outright accused Cohen of having lied under oath in August when he told a federal judge that it was at Trump's direction that he facilitated hush agreements for women who alleged to have had affairs with the President.

"He wasn't in trouble for what he did for me; he was in trouble for what he did for other people," Trump told the AP. "And what he was is also a public relations person. And now if he wants to try and get a little bit lighter sentence for what he did."

Trump's assertion that Cohen did not get in trouble for work he did for the President is downright false.

Cohen pleaded guilty in a Manhattan federal court to eight criminal counts in August, including tax evasion, making false statements to a bank and campaign finance violations. He admitted that "in coordination and at the direction of a candidate for federal office" -- Trump -- he kept information that could have harmed the President from becoming public during the 2016 election.

While the tax evasion and false statement charges were rooted in Cohen's taxi medallion business, the campaign finance charges were directly related to his work for Trump -- specifically, payments Cohen made or helped orchestrate that were designed to silence adult film star Stormy Daniels and former Playboy model Karen McDougal. Trump has denied having affairs with both women.

Cohen is scheduled to be sentenced in December and his plea deal didn't include a cooperation agreement. This means he can try to aid investigators in an effort to earn a reduced sentence.

Friends and associates of Cohen's have told CNN that over the past few months, Cohen initially looked for signs of support from his former boss. But he grew increasingly isolated -- and even distraught -- when it became clear that neither Trump nor the White House were coming to his defense.

And now, they say, protecting his family -- including minimizing legal bills -- is Cohen's singular focus.

"He's very resigned to doing the time. He's resigned to the fact that he's going to go to jail for some time," a source familiar with Cohen's thinking previously told CNN

The question now is whether Cohen can do anything to minimize jail time.

According to Davis, Cohen has provided the special counsel's office with "critical information" to aid their investigation into Russian collusion in the 2016 election.

In addition, Cohen and his attorney met Wednesday with federal prosecutors from the US Attorney's Office for the Southern District of New York and with officials from the New York Attorney General's office, according to people familiar with the matter. Both of those offices are continuing to investigate various aspects of Trump's family business and charitable organization, CNN has reported.

Cohen had been in contact with the New York state attorney general's office since shortly after his guilty plea -- potentially significant because the office has a civil lawsuit against the Trump Foundation and has been coordinating its investigation with the US attorney's office.

Trump Organization investigations

Another area where Cohen could prove to have useful information about his former boss: the innerworkings of the Trump Organization.

Cohen and his attorney Guy Petrillo have met with investigators and lawyers at the New York State Department of Taxation and Finance, according to a department spokesman. That office, CNN has reported, is conducting an inquiry into possible tax fraud at the Trump Organization based on evidence included in the August charges against Cohen from federal prosecutors. It is also examining whether the company kept accurate books and records, as the failure to do so is a state crime.

In a dramatic set of raids in the spring, investigators collected millions of documents, recordings and data from Cohen's home, hotel room and office. They included audio files that Cohen secretly made of his private conversations with Trump. The surreptitious recordings were particularly striking given the fact that Cohen made a name for himself over the years as Trump's most strident defender and loyalist.

For Trump, who famously values loyalty above all else and is wary of trusting even those in his inner-most circle, Cohen's evolution epitomizes the ultimate betrayal. And for Cohen, who is looking at the daunting possibility of jail time, turning his back on the President is likely the only feasible way to protect his family and salvage his reputation.

Sam Nunberg, a former Trump adviser who has also known Cohen for years, said the two men's public feud confirmed that under the surface, theirs was always a "relationship of convenience."

"Michael clearly doesn't feel that his loyalty was returned by the president and any impartial spectator would agree," Nunberg said. "I was shocked that he taped conversations with the President. In hindsight, I don't think he was as loyal as he pretended to be."