Well, this certainly will heat things up between the Senate and the Trump administration. From Sen. Dick Blumenthal (D-CT):

.@SenBlumenthal with a stark warning to Donald Trump. Jr should he fail to comply with Senate Intel subpoena: "If he fails to answer, he ought to be put in jail. If he fails to comply with a lawful subpoena, he has no privilege. Prison is the only answer." — Mariam Khan (@MKhan47) May 9, 2019

From colleague Jason Donner. Dem CT Sen Blumenthal on Senate subpoena for Donald Trump Jr: He ought to be held in contempt..If he fails to answer he ought to be put in jail. If he fails to comply with a lawful subpoena, he has no privilege. Prison is the only answer. — Chad Pergram (@ChadPergram) May 9, 2019

Good luck with that:

Lindsey Graham on Sen. Intel Trump Jr. subpoena: “If I were his lawyer I wouldn’t put him back into this circus. The freak show in the house, I'm not gonna let it come to the Senate so we gave Mueller two years and $25 million and it's over for me.” — Jason Donner (@jason_donner) May 9, 2019

And more Republican Senators have come to Trump Jr.’s defense:

I agree with Leader McConnell: this case is closed. The Mueller Report cleared @DonaldJTrumpJr and he’s already spent 27 hours testifying before Congress. Dems have made it clear this is all about politics. It’s time to move on & start focusing on issues that matter to Americans. https://t.co/11THs9LE0j — Senator Thom Tillis (@SenThomTillis) May 9, 2019

Chuck Grassley: "I think about the time that he [Trump Jr.] spent hours w/our staff & testimony…everything post the Mueller report coming out & its conclusions of no collusion or any crime. What more is there to do now? You'll have to ask the people of the Intelligence Cmte." — Jason Donner (@jason_donner) May 9, 2019

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Related:

'Source close to' Donald Trump Jr. RIPS into 'cowardly' Sen. Richard Burr over subpoena https://t.co/udGRz1WGDw — Twitchy Team (@TwitchyTeam) May 9, 2019