“It kind of puts him in a box,” said Matthew T. Sanderson, a campaign finance lawyer who has advised several Republican presidential hopefuls, including Senator Rand Paul of Kentucky and Rick Perry, the former Texas governor and energy secretary.
Mr. Giuliani, other campaign finance lawyers said, might have a hard time defending the assertion that all of his legal advice to Mr. Trump relates to his status as a candidate, given that much of the effort involves actions Mr. Trump has taken while serving as president. It also would add weight to the argument that the work Mr. Giuliani was doing on Ukraine was related to Mr. Trump’s re-election effort, two of the lawyers said
“If what he is doing for the president is for the campaign, how does he argue that the Ukraine corruption issue is not about the election?” asked Larry Noble, the F.E.C.’s former general counsel. “They are inconsistent arguments.”
In late November, a liberal watchdog organization, Citizens for Responsibility and Ethics in Washington, asked the Justice Department and the Office of Government Ethics to investigate the nondisclosure, saying it “seemingly violates federal law and undermines the integrity of the public financial disclosure program.”
“We’re not saying that President Trump can’t accept the free legal work,” said Virginia Canter, a lawyer for the group, which frequently sues the Trump administration. “What he does have to do is disclose the value and nature of any pro bono services.”
Mr. Clinton, who was under investigation by Congress and a special counsel for much of his time in office, raised more than $10 million through two funds to pay legal bills. Donations were capped at $1,000 in the first of the funds, and at $10,000 in the second.
During the his presidency, a conservative watchdog, Judicial Watch, claimed in a lawsuit that Mr. Clinton’s personal insurer was bending its coverage rules to pay some of his legal bills, saying it was trying to win “gratitude and favor” to create an agreeable “regulatory environment.” The suit did not succeed.
Ms. Clark, the law professor, said that Mr. Clinton and Mr. Trump were in parallel situations, in that both relied on donations to meet substantial legal costs, but reiterated that donations to the Clinton fund were capped at $10,000 a year. The value of a single day of Mr. Giuliani’s donated services would far exceed that. “There is no comparison in the size of the individual gifts,” she said.