Senators say Paul Manafort, Donald Trump Jr. will get subpoenas if they skip hearing

The Senate judiciary committee invited both men to appear next Wednesday.

“If they don’t voluntarily come, they will be subpoenaed,” Grassley told reporters Thursday morning.

Both men have come under increased scrutiny for a controversial meeting they had with a Russian lawyer in June of 2016 at Trump Tower.

Through an intermediary, Donald Trump Jr. set up a meeting with a “Russian government attorney” who had offered "official documents and information that would incriminate Hillary [Clinton]," according to emails Trump Jr. released from a conversations with Rob Goldstone, the publicist who offered to set up the meeting.

“If it’s what you say I love it especially later in the summer,” Trump Jr. wrote to Goldstone.

The Senate Judiciary Committee has also invited Glenn Simpson, a political operative and founder of the research firm Fusion GPS, to appear for the hearing next week. Simpson’s firm was reportedly associated with a pro-Russian lobbying campaign in Washington and has also been connected to the dossier of unverified allegations against Trump collected by his political opponents during the presidential campaign.

ABC's John Santucci contributed to this report.