WASHINGTON (AP) — President Donald Trump's most prominent GOP critics on Capitol Hill are close to completing their Senate careers, raising the question of who — if anyone — will take their place as willing to publicly criticize a president who remains popular with nearly 9 in 10 Republican voters.

Sens. Jeff Flake of Arizona and Bob Corker of Tennessee engaged in a war of words with the president on myriad issues over the past 18 months, generating headlines and fiery tweets from a president who generally insists on getting the last word. Those battles put them on the outs with many in their own party, and they paid a price. Both decided to retire rather than take on a difficult re-election campaign.

Flake was far and away Trump's most consistent critic among Senate Republicans. Corker weighed in less often, but his description of the White House as an "adult day care center" rankled the president, who dubbed him "Liddle' Bob Corker." The feud continued as Corker headed for the exits, with Trump asserting that Corker's promise to serve only two terms was not the real reason he retired. Rather, Corker "wanted to run but poll numbers TANKED when I wouldn't endorse him," Trump tweeted.

Corker replied: "Yes, just like Mexico is paying for the wall... #AlertTheDaycareStaff."

One possible voice of dissent could come from Utah Sen.-elect Mitt Romney. In a Washington Post op-ed Tuesday, Romney wrote that Trump's "conduct over the past two years ... is evidence that the president has not risen to the mantle of the office."

Romney praised some of Trump's policy decisions, but added: "With the nation so divided, resentful and angry, presidential leadership in qualities of character is indispensable. And it is in this province where the incumbent's shortfall has been most glaring."

Romney has had his public run-ins with the president.

Throughout his Senate campaign, Romney insisted that he would agree with Trump on some issues and not be shy about disagreeing on others. Romney appears to have more room with GOP voters in Utah to take on the president. Most voters in Utah — 64 percent — would like to see the senator confront the president, according to data from AP VoteCast, a survey of midterm voters.

Sen. Ben Sasse of Nebraska has frequently criticized Trump for what he considers the president's uncivil rhetoric. Sasse has also said he "regularly" considers leaving his party and becoming an independent. He is up for re-election in 2020 and has said he'll decide by the summer whether to seek a second term. It would be tricky terrain for Sasse to publicly battle with Trump, who won the state in 2016 by 25 percentage points.

Rory Cooper, a GOP strategist who helped lead the "Never Trump PAC" during the 2016 Republican primary, said publicly criticizing the president makes Republican votes back home unhappy and earns the ire of the president. Meanwhile, Democratic voters and the media give them too little credit, he said.

"There is not an incentive structure for senators who disagree with or oppose the president to speak out right now, but that could change if the (Robert) Mueller investigation continues to move in the direction it has been or the economy churns negative," Cooper said.

Senate Republicans chafe at the notion they are unwilling to take on a president whose statements and policy positions often run counter to traditional conservative positions.

Sen. Mike Rounds, R-S.D., said he raised his concerns about trade policy privately with the president.

"I want results," Rounds said, "instead of hardening positions."

Rounds said he rode with Trump after meeting him at the airport when the president visited South Dakota during the midterm election season, telling him the state's soybean farmers were facing losses of some $500 million because of retaliatory tariffs. He said Trump told him: "We're going to have a better deal for them. If they hang with me, we're going to make this better."

Rounds said his job is to make things better and "that doesn't mean I have to be out there in front fighting with someone."

Sen. David Perdue, R-Ga., said "there's all sorts of active disagreements that go on" with the White House behind the scenes.

"I support the president as well as anybody. Behind closed doors, there are things on tariffs and things like that where we've offered a differing opinion. But I support this president in terms of what he's trying to do. This agenda is working," Perdue said, citing strong economic growth and low unemployment.

Democratic Sen. Dick Durbin, D-Ill., said he suspects the 2020 elections will prompt more Republican senators to confront Trump when they disagree with him, and that may already be happening, citing recent actions on Saudi Arabia.

The Senate passed a measure that blamed Saudi Crown Prince Mohammed bin Salman for the killing of journalist Jamal Khashoggi and called on Riyadh to "ensure appropriate accountability." Senators also passed a separate measure calling for the end of U.S. support for the Saudi-led war in Yemen. The resolutions showed senators seeking to assert oversight of Trump administration foreign policy and the relationship with Saudi Arabia.

"Almost half of their caucus is up for re-election. They just saw what happened in 2018," Durbin said of Republicans, who lost the House majority to Democrats. "I think, once they do polling back home, not all of them, but many of them will find that independence is being rewarded."