President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE on Tuesday endorsed Sen. Ben Sasse Benjamin (Ben) Eric SasseWhy a backdoor to encrypted data is detrimental to cybersecurity and data integrity McEnany says Trump will accept result of 'free and fair election' McConnell pushes back on Trump: 'There will be an orderly transition' MORE (R-Neb.) for his reelection bid in 2020, sending a message of support for one of his more outspoken Republican critics on Capitol Hill.

“Senator Ben Sasse has done a wonderful job representing the people of Nebraska. He is great with our Vets, the Military, and your very important Second Amendment. Strong on Crime and the Border, Ben has my Complete and Total Endorsement!” Trump tweeted.

Senator Ben Sasse has done a wonderful job representing the people of Nebraska. He is great with our Vets, the Military, and your very important Second Amendment. Strong on Crime and the Border, Ben has my Complete and Total Endorsement! — Donald J. Trump (@realDonaldTrump) September 11, 2019

Sasse, who officially announced his reelection bid in August, has been one of the few GOP lawmakers on Capitol Hill to criticize the White House and had previously floated a primary challenge against Trump next year.

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The Nebraska Republican began hammering Trump during the 2016 campaign and has ripped him during his White House tenure for his “amoralistic” rhetoric as well as his trade and foreign policy decisions.

Trump has responded to the criticism, tweeting early in 2016 that “the great State of Nebraska can do much better than @BenSasse as your Senator.”

The great State of Nebraska can do much better than @BenSasse as your Senator. Saw him on @greta - totally ineffective. Wants paid for pols. — Donald J. Trump (@realDonaldTrump) January 30, 2016

While Trump continues to enjoy a sky-high approval rating within the Republican Party, Sasse has been protected against a wave of threats to his seat partially due to his solidly conservative voting record and his backing from the National Republican Senatorial Committee.

The American Conservative Union rated Sasse in 2017 as the Senate’s second-most conservative member after Sen. Rand Paul Randal (Rand) Howard PaulRon Paul hospitalized in Texas The Hill's Morning Report - Sponsored by Facebook - Trump previews SCOTUS nominee as 'totally brilliant' Rand Paul says he can't judge 'guilt or innocence' in Breonna Taylor case MORE (R-Ky.), and he has emerged as one of the most vocal opponents of abortion, attempting to force a vote in February on the Born-Alive Abortion Survivors Protection Act, which would protect newborns who survive abortions.

Thus far only Matt Innis, the former chairman of the Lancaster County Republican Party, has announced a primary challenge to Sasse.

Sasse won his first term in 2014 by 33 points, and his seat is rated as “Solid Republican” by The Cook Political Report, a nonpartisan election handicapper.