President Trump's son and former campaign chairman are both expected to meet with the Senate Judiciary Committee this week, but in a move that's irritated some Democrats, they will reportedly not be put under oath to answer the panel's questions.

Likewise, Trump's son-in-law and senior adviser Jared Kushner was not expected to take an oath during his Monday meeting with the Senate Intelligence Committee, The Washington Post reported.

Legally speaking, taking an oath is closer to a symbolic gesture than a move needed to keep witnesses honest. That didn't stop Democrats from getting up in arms about the decisions.

"How do we know Crooked Jared Kushner is lying? Because he refused to make his statements under oath," said Rep. Ted Lieu, D-Calif., on Twitter.

Sen. Al Franken, D-Minn., sits on the Judiciary Committee, but a Sunday interview on CNN was the first he'd heard that Paul Manafort and Donald Trump Jr. wouldn't be administered an oath before answering questions.

"That's not good enough. They should be under oath," Franken told CNN's Jake Tapper.

Franken also noted that in an interview on July 11 with Fox News' Sean Hannity, Trump Jr. said specifically that he would be willing to testify under oath.

"He didn't say he would testify publicly, but 'under oath' he said," said Franken. "So he should definitely do that."

The Senate Judiciary Committee had originally asked Trump Jr. and Manafort to testify at a public hearing on Wednesday, but instead they will have closed-door interviews, with the possibility of future public testimony.

Committee Chairman Chuck Grassley, R-Iowa, noted, "it's a crime to lie to congress" regardless of whether a person is under oath. He later defended the committee's process, tweeting on Monday that such interviews help with the accountability of future public testimony. Plus, he said, an interview with his staff and the staff of Sen. Dianne Feinstein, D-Calif., "is no walk in the park."

Federal law prohibits lying in these settings, even without an oath, Rutgers University law professor Stuart Green told NPR. The law — often referred to as the False Statements Act — prohibits making "any materially false, fictitious, or fraudulent statement or representation" within the jurisdiction of the executive, legislative, or judicial branches of government.

It is normally used in the context of law enforcement, like an interview with an FBI investigator, but the law also applies to congressional testimony.

Just because you haven't taken an oath, "it's not like you can lie with impunity before the committee," Green said.

Taking an oath does, however, raise the stakes.

Should someone lie knowingly without taking an oath, they would be breaking the false statements statute. Should they lie after taking an oath, they run the risk of also being charged with perjury.

Yet both of those kinds of charges are "extremely unusual," Green noted. Because of that, the importance of the oath comes largely from its perceived solemnity, he said. The photo of a witness with her or his hand on a Bible on the front of a newspaper, next to the words that were said, gives those statements a certain weight potentially more powerful than the actual legal ramifications.

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