Mitt Romney may not endorse Donald Trump for re-election in 2020, partly because the 2012 Republican nominee thinks political endorsements are not “worth a thimble of spit”.

That said, the Utah senator remains happy to endorse his choice for president in 2016. Then, having called his party’s nominee “a phoney, a fraud”, he voted for his wife, Ann. This week, Romney said: “I still think she’s doing a fine job.”

The former Massachusetts governor was elected to the Senate last year, after attempts at a rapprochement with Trump produced an unsuccessful bid to be named secretary of state that generated embarrassing public appearances.

Trump endorsed Romney for Congress. Nonetheless the senator has emerged as a leading critic, if inconsistently and largely if not entirely regarding foreign policy.

In January, Romney wrote a scathing column for the Washington Post in which he lamented Trump’s behaviour, writing: “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.”

But he recently declined to join the Michigan representative Justin Amash in calling for Trump’s impeachment from the Republican side of the aisle.

Trump faces one declared challenger for the 2020 nomination, former Massachusetts governor Bill Weld. Last week Maryland’s governor, Larry Hogan, who was courted by anti-Trump conservatives and openly considered a run, said he would not attempt to take Trump down.

Romney’s niece is chair of the Republican National Committee, which has faced criticism for supposedly seeking to block a primary challenge. As she runs a party firmly under Trump’s sway, Ronna McDaniel has ceased using Romney as her middle name.

Romney spoke to reporters on Friday at the E2 Summit, a gathering of politicians, donors and business figures in Park City, Utah. Romney hosts the event, which is chaired by former House speaker Paul Ryan, another who did not meet with noticeable success when attempting to oppose, contain or support the president.

“I don’t think endorsements are worth a thimble of spit,” Romney said. “I wouldn’t be surprised if I stay out of the endorsements.”

He did not say who he thought would win the Democratic nomination, though he did say former vice-president Joe Biden would be a tougher challenger than his nearest rival in the polls, the Vermont senator Bernie Sanders.

He added that he thought “the attitude here has got to be the same as across the country, which is the president will surely be the Republican nominee and an incumbent in a growing economy is more likely to win than to lose. But it’s not a sure thing.”

Speaking before Trump announced a deal with Mexico on immigration and the withdrawal of threatened tariffs, Romney said: “Punishing Mexico in some way is, in my opinion, a very bad idea. It’s also expensive for Americans.”

But he said he did support Trump’s imposition of tariffs on Chinese goods, which critics have said will similarly hurt American consumers.

“President Trump was right to push back hard on China, and I support that effort,” Romney said. “I don’t think there’s much recognition in the United States about the long-term issues with China. We haven’t laid out a real comprehensive strategy to say, ‘How do we up our game dramatically?’”

The Deseret News reported that the former Australian prime minister Kevin Rudd, appearing alongside Romney, was greeted by laughter when he said Trump was “right out there. He’s kind of Teddy Roosevelt on speed.”