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.

 -- Sen. Chuck Grassley, R-Iowa, the chairman of the Senate Judiciary Committee, is threatening to issue subpoenas to compel Paul Manafort and Donald Trump Jr. to testify before Congress if they do not cooperate with the panel’s ongoing investigation and appear for a public hearing next week.

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

Yesterday, the panel formally invited both men to appear before the committee next Wednesday for a hearing on 2016 presidential election interference. Committee investigators also requested a litany of documents from both men related to campaign contacts with Russian officials.

Manafort, President Donald Trump’s former campaign manager, and Trump Jr., the president’s eldest son, are still reviewing the Judiciary Committee's requests.

“Am I concerned? No,” said Dianne Feinstein, D-California, the committee’s top Democrat. “I’m not concerned because if they don’t they will be subpoenaed.”

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.

Trump Jr. invited Manafort and brother-in-law Jared Kushner, now a senior White House adviser, to meeting with Natalia Veselnitskaya, the Russian lawyer. Kushner is set to appear for an interview behind closed doors with the Senate Intelligence Committee next week.

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.

Grassley has written to the Justice Department with concerns that Fusion GPS may have violated U.S. lobbying laws.

ABC's John Santucci contributed to this report.