Outgoing Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeRepublican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style Bush endorsing Biden? Don't hold your breath MORE (R-Ariz.) on Wednesday said that the GOP is "in for a rough ride" in the 2020 elections if the party doesn't make changes before then.

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Flake, appearing on CNN's Anderson Cooper 360, added that Republicans will soon need to decide if "they want to be tethered" politically to President Trump Donald John TrumpBarr criticizes DOJ in speech declaring all agency power 'is invested in the attorney general' Military leaders asked about using heat ray on protesters outside White House: report Powell warns failure to reach COVID-19 deal could 'scar and damage' economy MORE.

"And that's going to be what's really going to be interesting in the coming months, because if you look at the midterms, the Senate was very difficult for Republicans to lose, but losing 40 seats in the House and looking at where we are in terms of the suburbs, in terms of women and minorities, we're in for a rough ride in 2020, if we don't make some changes," he said.

Outgoing Republican Sen. @JeffFlake praises incoming GOP Sen-elect @MittRomney for his scathing op-ed directed at President Trump, says he was not surprised President Trump dragged his name into the spat. https://t.co/zjKIShSmps pic.twitter.com/JI4Ymig8uw — Anderson Cooper 360° (@AC360) January 3, 2019

Republicans gained two Senate seats in the midterm elections but Democrats easily regained control of the House.

During the interview with Cooper, Flake, who is a frequent critic of President Trump, also called the GOP "the president's party."

"This is very much the president's party, when you look at the base and look at those who vote in Republican primaries I think that is clear," he said.

"That won't always be the case, but it certainly is now," Flake continued. "And so somebody who wants to run for reelection two years from now has to decide, you know, where they are."

Flake reiterated in the interview that he hopes a Republican brings a primary challenge against Trump in 2020, though he didn't say whether he would be the one to do that.

The retiring senator also praised Sen.-elect Mitt Romney Willard (Mitt) Mitt RomneyThe Memo: Warning signs flash for Trump on debates GOP votes to authorize subpoenas, depositions in Obama-era probe Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE (R-Utah) for writing an op-ed in The Washington Post this week that was critical of Trump's character. Romney wrote in the op-ed that Trump has not "risen to the mantle" of the presidency.

"I thought it was a great op-ed that was written," Flake said. "I think it said things that need to continually be said. And so I applaud Mitt Romney for saying that."