WASHINGTON (AP) — President Donald Trump’s repeated prodding of the Ukrainian president to investigate Joe Biden and his son could amount to an illegal request for a campaign contribution from a foreign citizen.

Federal law states it is illegal to “knowingly solicit, accept, or receive from a foreign national any contribution or donation.” Trump’s request to President Volodymyr Zelenskiy was not for campaign cash, but what’s referred to as an “in kind” contribution that would arguably be of more value — damaging information that could be weaponized against Biden, a potential 2020 rival.

That’s likely to be among the issues House Democrats focus on as they pursue an impeachment inquiry into efforts by Trump and his attorney Rudy Giuliani. The former New York mayor spearheaded Trump’s effort to obtain information on Biden and his son Hunter, who did work for a Ukrainian gas company while his father was vice president.

“It turns on a basic question,” said Larry Noble, a former general counsel to the Federal Election Commission who is a Trump critic. “Is it legal for the president of the United States to ask a foreign country to intervene in our election to help him and investigate his potential opponent? And I think it is clearly illegal.”

Trump has said he did nothing wrong. Justice Department prosecutors have determined Trump did not violate campaign finance law, including a prohibition on accepting campaign contributions or a “thing of value” from foreign governments. A department official said prosecutors made the determination based on the elements of the crime and did not consider the department’s policy prohibiting the indictment of a sitting president.

That makes impeachment the only likely avenue to pursue.

Giuliani, however, doesn’t enjoy the same immunity and could be charged for his role, legal experts say. Giuliani did not immediately respond to a request for comment Wednesday evening.

The drama unfolding in Congress revisits a central issue from special counsel Robert Mueller’s investigation: Did Trump campaign officials break the law by “knowingly” requesting, accepting or receiving a donation from a foreign national?

Mueller said “no” because it was difficult to tell whether they were aware of the law when Trump’s son and several advisers held a meeting with Russians promising dirt on Hillary Clinton. This time that would be a far more difficult argument to make after Trump has faced repeated questions in recent months over his willingness to accept foreign help.

One day after Mueller told Congress it was hard to prove his awareness of the law, Trump was on the phone with Zelenskiy seeking assistance digging into allegations against the Bidens, which have not been substantiated.

“There’s a lot of talk about Biden’s son,” Trump said.

At one point in the conversation, he said, “I will have Mr. Giuliani give you a call and I am also going to have Attorney General (William) Barr call and we will get to the bottom of it.”

So just how valuable is the information Trump was seeking?

In the days before the call, Trump ordered advisers to freeze $400 million in military aid for Ukraine — prompting speculation that he was holding out the money as leverage for the information. Trump has denied that charge, but acknowledged he blocked the funds, later released.

Trump has sought to implicate Biden and his son in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading the Obama administration’s diplomatic dealings with Kyiv. Though the timing raised concerns among anticorruption advocates, there has been no evidence of wrongdoing by either the former vice president or his son.

“Given the context of the call, President Trump created an implicit understanding that U.S. support for Ukraine and taxpayer-funded security aid to Ukraine was hanging in the balance,” said Trevor Potter, a Republican former FEC commissioner who is now president of the nonpartisan Campaign Legal Center.

Trump has angrily denounced the impeachment inquiry as “presidential harassment” and insisted he did nothing wrong because there was no “quid pro quo.”

“This is nothing more than a continuation of the Greatest and most Destructive Witch Hunt of all time!” he tweeted.

But “you don’t need a quid pro quo” for it to be illegal, said Noble.

Aside from Trump’s request to Zelenskiy, there are other campaign finance issues that could carry civil or criminal penalties for others involved in the effort — like whether someone footed the bill for work done by Giuliani, who has said he is not compensated.

Giuliani’s actions on Trump’s behalf could be construed as political activity, but there are no records in FEC filings of him getting paid. If he were compensated or incurred expenses that were paid from outside the campaign, that would likely need to be reported as a contribution, Noble said.

Depending on the amount of money involved, a violation could include civil penalties and, in some cases, jail time.

Still, establishing that the effort violated campaign finance law will not be an easy task, said Dan Petalas, a former FEC attorney who once gave a $250 donation to a Democrat and is now in private practice.

“It certainly raises a question,” he said. “It really will turn on a better picture of the facts and connecting the dots. It is just so outside the norm.”

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Associated Press writers Eric Tucker and Michael Balsamo contributed to this report.