President Trump's personal attorney Rudy Giuliani said Wednesday he has obtained legal representation from commercial and criminal litigation lawyer Robert Costello, as well as Eric Creizman and Melissa Madrigal from the Pierce Bainbridge firm.

The big picture: Giuliani is a key player in the Trump-Ukraine saga. He has continually campaigned for Ukrainian officials to investigate the Bidens and to dig into the origins of the Russia probe. House impeachment committee members and several witnesses have accused Trump of withholding military aid to Ukraine to pressure its government to announce these investigations.

Republicans and Democrats familiar with the closed-door testimony in the House impeachment inquiry tell Axios' Alayna Treene that Giuliani and his Ukraine activity have been a unifying thread for the witnesses.

New York federal prosecutors launched a criminal investigation into Giuliani to determine if he "broke lobbying laws in his dealings in Ukraine," the New York Times reports.

Background: Former Trump attorney Michael Cohen used 2018 emails from Costello as evidence for his claim that a presidential pardon "was dangled" before he cooperated with federal prosecutors, CNN reports.

Costello told Cohen in April last year that he could "sleep well tonight" since he had "friends in high places," per CNN.

Lev Parnas, one of Giuliani's foreign-born associates to be indicted on campaign finance charges, said via his lawyer on Monday that he would cooperate in the House's impeachment inquiry.

Go deeper: Amid Ukraine scandal, Trump officials worry about Rudy Giuliani's financial motives