Sen. Jeff Flake (R-AZ) called President Donald Trump’s proposed border wall “a bit, you know, out there,” on Thursday, the latest rebuke from Arizona’s junior senator of a President who has targeted him with sharp attacks.

“This notion of a 2,000-mile wall has always been just — for anybody who spends time on the border — just a bit, you know, out there,” he said in an interview with Phoenix, Arizona TV station KNXV.

“My position is I work with the President, vote with the President when I think he’s right, and I oppose him when I think he’s wrong,” Flake added separately.

Trump threatened to shut down the government if Congress did not appropriate funds for his proposed wall on the U.S.-Mexico border in a campaign rally in Phoenix on Tuesday. But on Thursday, White House press secretary Sarah Huckabee Sanders would not say that Trump was willing to shut down the government, insisting only that “the President’s committed to making sure this happens and we’re going to push forward.”

Asked about Trump’s erstwhile catchphrase that Mexico would pay for the wall, a promise he’s avoided recently, Sanders said “he hasn’t said they’re not, either.”

Flake and Trump have butted heads repeatedly in recent days.

In a book released earlier this month, “Conscious of a Conservative,” Flake said the Republican Party had betrayed its principles in backing Trump. On Wednesday, he said Trump was “inviting” a primary challenger for the 2020 Republican presidential nomination with his governing style.

Trump reportedly met Tuesday with Republicans considering challenging Flake in the 2018 Senate primary, and has called him out on Twitter:

Great to see that Dr. Kelli Ward is running against Flake Jeff Flake, who is WEAK on borders, crime and a non-factor in Senate. He's toxic! — Donald J. Trump (@realDonaldTrump) August 17, 2017

Phoenix crowd last night was amazing – a packed house. I love the Great State of Arizona. Not a fan of Jeff Flake, weak on crime & border! — Donald J. Trump (@realDonaldTrump) August 23, 2017

Even if Trump believes Flake is “weak on crime & border,” the senator has voted for Trump’s priorities more than 90 percent of the time, according to a tally by FiveThirtyEight.

H/t Politico