The U.S. Senate judiciary committee plans to question the former chair of the Trump campaign and will subpoena him if necessary, the panel's Republican chair said Wednesday.

Senator Chuck Grassley of Iowa said he and the committee's top Democrat, Dianne Feinstein of California, have agreed to try to bring Paul Manafort before the panel for questioning about the government's enforcement of a law requiring registration of foreign lobbyists. Feinstein's office confirmed that they plan to question him.

Manafort would unquestionably be asked about his participation in a Trump Tower meeting with a Russian lawyer last summer with President Donald Trump's eldest son and son-in-law, where the purpose was to hear potentially damaging information about Hillary Clinton from a Russian lawyer.

Manafort disclosed the meeting in a package of information he provided to the Senate and House intelligence committees, which have been investigating potential co-ordination between Russia and the Trump campaign, as is Robert Mueller, the former FBI director appointed by the Justice Department as the special counsel.

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President Donald Trump's son-in-law, Jared Kushner, left, and son Donald Trump Jr. attended the meeting with the Russian lawyer at Trump Tower with Manafort. (Rick Wilking/Reuters)

"Obviously it would be appropriate for anybody to get into anything that went on at that meeting, and he was at that meeting," Grassley told Iowa reporters.

A person close to Manafort said that he hasn't yet received a letter from the Senate judiciary committee about a possible interview. The person spoke on condition of anonymity to discuss Manafort's private interactions with the committee.

'Greatest witch hunt in political history'

Donald Trump Jr. disclosed on Twitter a series of emails on Tuesday that revealed his eagerness to hear negative material on Clinton from a Russian lawyer. The exchange showed Trump Jr. conversing with a music publicist who wanted him to meet with a "Russian government attorney" who supposedly had dirt on Clinton as "part of Russia and its government's support for Mr. Trump." He was told the Russian government had information that could "incriminate" Clinton and her dealings with Russia.

 "If it's what you say I love it especially later in the summer," Trump Jr. said in one email response.

In an interview before departing Wednesday evening for France, Trump told Reuters that he didn't know about the meeting "until a couple of days ago when I heard about this." He also said that he didn't fault his son for attending. "I think many people would have held that meeting," he said.

Trump also said in a tweet that his son was "open, transparent and innocent" and again dismissed the ongoing Russia investigation as the "greatest Witch Hunt in political history."

But Trump also refused to dispute an assertion made by Russian President Vladimir Putin that Trump believed him when he said that Russia hadn't meddled in the election during their lengthy meeting in Germany last week.

"Look. Something happened and we have to find out what it is, because we can't allow a thing like that to happen to our election process," Trump said. But he added of Putin: "Somebody did say if he did do it you wouldn't have found out about it. Which is a very interesting point."

Trump also questioned whether Putin was really gunning for his victory, as U.S. intelligence agencies have concluded.

"It's really the one question I wish I would have asked Putin: Were you actually supporting me?" Trump said.

The president's attorney, Jay Sekulow, said in an interview with NBC's Today that Trump Jr. did not violate any laws by accepting the meeting. He said the president had not been aware of Trump Jr.'s June 2016 meeting and didn't find out about his son's email exchange until "very recently."

Sekulow said the president was not being investigated by Mueller. "I would know a little bit about it. I'm one of the lawyers," Sekulow told ABC's Good Morning America.

As the emails reverberated across the political world, Trump Jr. defended his actions in an interview with Fox News, blaming the decision to take the meeting on the "million miles per hour" pace of a presidential campaign and his suspicion that the lawyer might have information about "underreported" scandals involving Clinton. Trump Jr. said the meeting "really went nowhere" and that he never told his father about it because there was "nothing to tell."

"In retrospect I probably would have done things a little differently," Trump Jr. said.

Investigations into possible collusion between Russia and the Trump campaign have shadowed the White House for months. 

Still Trump remains defiant. "There was zero co-ordination" between his campaign and Russia," he told Reuters, adding: "It's the dumbest thing I've ever heard."

With files from Reuters