Sen. Al Franken Alan (Al) Stuart FrankenGOP Senate candidate says Trump, Republicans will surprise in Minnesota Peterson faces fight of his career in deep-red Minnesota district Getting tight — the psychology of cancel culture MORE (D-Minn.) said on Sunday that Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE Jr. and former campaign chairman Paul Manafort should testify under oath.

During an interview on CNN's "State of the Union," Franken was asked whether it was good enough for the two to testify behind closed doors and not under oath.

"No, that's not good enough," Franken said. "It should be under oath."

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Franken said he would like to ask questions about whether they had any other meetings with the Russians.

The comments come after lawmakers announced last week that Trump Jr. and Manafort reached a deal with the Senate Judiciary Committee to avoid appearing at a public hearing.

Manafort and the president's eldest son are the focus of new attention after reports that they attended a meeting, along with Trump's son-in-law Jared Kushner, with a Russian lawyer. An email setting up the meeting promised damaging information on Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE that would be provided by the Russian government.

Sens. Chuck Grassley (R-Iowa) and Dianne Feinstein (D-Calif.) said in a joint statement that they would not immediately subpoena Trump Jr. or Manafort to testify at the Wednesday hearing.

"Both Donald Trump Jr. and Paul Manafort, through their attorneys, have agreed to negotiate to provide the committee with documents and be interviewed by committee members and staff prior to a public hearing," the lawmakers said.