"If he comes before our committee - and we'll subpoena him if necessary - then it would be appropriate for anybody to get into anything that went on at that meeting", Grassley said.
The emails are the first documentary evidence that a top Trump associate took a meeting to hear damaging information about Clinton with the understanding that it was connected to a Russian government effort to help Trump in the 2016 election.
Trump Jr. has said that while he would have probably handled the meeting request differently now, he believes he did nothing wrong by hearing out someone offering information about a political opponent during a heated political campaign.
The letter questioned how Natalia Veselnitskaya - the Russian lawyer who met with the Trump campaign last summer - was allowed to travel to the United States, despite past attempts where she was denied a USA visa. Congressional committees are investigating, as is Special Counsel Robert Mueller.
The senator from the U.S. state of Iowa added that if necessary, he will subpoena Manafort.
'But the real way I feel comfortable inviting him is ever since President Trump was elected it seems like every conversation that has come from somebody in the family, where there's been some sort of issue, they've seemed always to be very, very open, ' Grassley said.
In response to calls for him to testify before the intelligence panel, Trump Jr. tweeted Monday that he was "happy to work with the committee to pass on what I know".
The lawmakers spoke one day after Donald Trump Jr. disclosed on Twitter a series of emails that revealed his eagerness to hear negative material on Clinton from a Russian lawyer.
In the emails, Trump Jr. was told by an interlocutor that he could get "very high level and sensitive information" that was "part of Russian Federation and its government's support for Mr. Trump".
Trump Jr. told Fox News' Sean Hannity Tuesday night that he would be willing to testify under oath about the meeting, which he said yielded no useful information and instead focused on adoptions.
Trump says the woman is a private attorney who has "roamed the hallways" of the U.S. Congress. That email led to the June 9, 2016 meeting, at which Trump Jr. later said no meaningful information was provided. "There was zero coordination" between his campaign and Russian Federation", he told Reuters, adding: "It's the dumbest thing I've ever heard".