Michael Cohen, former personal lawyer to U.S. President Donald Trump, arrives to a House Oversight Committee hearing in Washington, D.C., U.S., on Wednesday, Feb. 27, 2019.

President Donald Trump is in Vietnam to hold nuclear talks with North Korean dictator Kim Jong Un, so it will be up to his Republican allies on Capitol Hill to defend him in hearings with his former personal lawyer, Michael Cohen.

With Cohen expected to tell lawmakers this week about alleged criminal conduct committed by Trump while in office, GOP lawmakers plan to push back on his claims by attacking Cohen's credibility and trustworthiness.

Cohen, 52, is set to head to prison for a three-year sentence starting in May, after pleading guilty to campaign finance violations and criminal tax evasion brought by federal prosecutors in Manhattan, as well as a charge of lying to Congress lodged by special counsel Robert Mueller as part of his ongoing Russia probe.

But before his incarceration, Cohen will appear before the House Oversight Committee on Wednesday for a public hearing, where he will reportedly share evidence alleging that Trump committed criminal conduct since becoming president.

Cohen will also discuss Trump's finances, including Trump's alleged involvement in a hush-money deal brokered by Cohen before the 2016 election to silence porn star Stormy Daniels, who claims she had a brief affair with Trump years earlier, a source with knowledge of Cohen's preparations told NBC News. Trump denies the affair.

Cohen will also testify behind closed doors before the Intelligence Committees of the Senate and House on Tuesday and Thursday, respectively.

Democrats on the committees are reportedly expected to grill Cohen on a variety of topics over the course of the week, including Mueller's probe of Russian election interference and possible Trump-campaign collusion. That could encompass the scrapped Trump Tower Moscow dealings or the June 2016 Trump Tower meeting with a Kremlin-connected Russian lawyer.

"He's the only person that I know of who has accused this president of a crime," House Oversight Chairman Elijah Cummings, D-Md., said of Cohen. "And so I think it's only fair to the president and to Michael Cohen and to the public that he come forward so that they'll have an opportunity to observe his demeanor, Republicans will be able to ask him questions, just like in a cross examination, and then they can make their own judgments."

His committee's Republican members, however, aren't planning on learning much of anything from Cohen.

Rather, the panel's GOP members are mainly poised to attack Cohen's credibility during the Wednesday hearing, a congressional aide familiar with the committee's minority told CNBC.

Those lawmakers are likely to highlight the Southern District of New York's sentencing memo in Cohen's case, the aide said, in which prosecutors challenged the former Trump Organization executive's defense that he had cooperated extensively with law enforcement and had reformed his outlook.

"For all of Cohen's outward rectitude, he has lived a double life," prosecutors argued in that memo. "While Cohen has submitted letters describing his good nature, the evidence collected and witnesses interviewed in this investigation paint a decidedly different picture — a picture of someone who was threatening and abusive when he wanted to get his way."

The Republicans will further argue that Cohen's presence before the committee in a public hearing — among the first such events since Democrats took majority control in January — damages the credibility of the panel.

"Cummings promised us fact-based oversight, and we're bringing in a convicted felon," the aide said. "It kind of makes us not have as much credibility as we started out with."

The aide added that the minority panel wasn't especially interested in probing new facts from Cohen, whom they sought to establish as not credible. "As far as GOP members gleaning new information from Mr. Cohen, he's someone that we don't really trust and we can't really believe anything that he has to say," the aide said.

After FBI agents raided Cohen's properties in April, Trump decried the treatment of his once-loyal fixer, tweeting at the time that Cohen was "a fine person with a wonderful family" who "I have always liked & respected."

But after Cohen cut a deal with prosecutors, Trump's view of him soured.

Trump TWEET If anyone is looking for a good lawyer, I would strongly suggest that you don't retain the services of Michael Cohen!

In December, Trump tweeted that Cohen was a "Rat" who was "just trying to get his sentence reduced."

There were no insults or denunciations from Trump's Twitter account when Cohen arrived on Capitol Hill for his first closed-door testimony Tuesday morning, however. Trump had just arrived in Vietnam for a meeting in Hanoi with Kim.

Hopes are high that Trump's second summit with Kim could push the ball forward on denuclearizing the isolated state or possibly lead to a declaration that formally ends the 1950-53 Korean War.

Trump TWEET Meeting for breakfast with our Nation's Governors - then off to Vietnam for a very important Summit with Kim Jong Un. With complete Denuclearization, North Korea will rapidly become an Economic Powerhouse. Without it, just more of the same. Chairman Kim will make a wise decision!

With Trump himself focused on Kim, White House press secretary Sarah Huckabee Sanders released a statement bashing Cohen before his Senate testimony.

"Disgraced felon Michael Cohen is going to prison for lying to Congress and making other false statements. Sadly, he will go before Congress this week and we can expect more of the same," Sanders said in a statement. "It's laughable that anyone would take a convicted liar like Cohen at his word, and pathetic to see him given yet another opportunity to spread his lies."

The Senate committee's GOP majority echoed that stance ahead of Cohen's hearing.

Manu Raju TWEET Senate Intelligence Committee Chairman Richard Burr on what he wants to hear from Michael Cohen, and he said: "The truth." Asked if he could trust Cohen, and Burr noted Cohen's questionable track record.

— CNBC's Dan Mangan contributed to this report.