At a private event in Virginia, senior Republican senator Mitt Romney delivered a stark warning about the threat to US democracy in light of Russia’s invasion of Ukraine.

“We are really the only significant experiment in democracy, and preserving liberal democracy is an extraordinary challenge,” the Utah senator and former Massachusetts governor and Republican presidential nominee said, according to attendees at the event in McLean, Virginia, who spoke to CBS News.

The event was held to raise funds for Liz Cheney, the Wyoming Republican whose opposition to Donald Trump over his attempt to overturn his election defeat and the January 6 riot has attracted a far-right challenger backed by the former president and the House Republican leader, Kevin McCarthy, in her primary this year.

Cheney’s father is Dick Cheney, the former congressman, defense secretary and vice-president to George W Bush. CBS said he and other Republican establishment figures attended the event on Monday, which raised more than $526,000.

Romney, CBS reported, cited the Russian invasion of Ukraine and said the Russian president, Vladimir Putin, was following an authoritarian playbook “rehearsed time and time again, over the many thousands of years of world history”.

Romney reportedly said he had a chart in his Senate office tracing 4,000 years of the rise and fall of civilizations. CBS reported: “From the Mongol Empire to the Roman Empire, Romney said, autocracy is the chart’s ‘default setting’.”

Critics said Trump was too close to Putin when in office. He has repeatedly praised the Russian leader since the invasion of Ukraine began. He has also condemned the war.

Romney said: “What has kept us from falling in with the same kind of authoritarian leader as Vladimir Putin are the strengths of our institutions, the rule of law, our courts, Congress and so forth.

“People of character and courage have stood up for right at times when others want to look away. Such a person is Liz Cheney.”

Cheney voted against impeaching Trump for withholding military aid to Ukraine in an attempt to extract dirt on political rivals. She voted for impeachment for inciting the Capitol attack.

Romney was the only Republican senator to vote to convict Trump in both his impeachment trials. Trump was twice acquitted, although six other Republicans joined Romney in voting to convict over the attack on Congress.

Trump is in legal jeopardy over election subversion and his business affairs but he dominates his party, topping polls of potential nominees for 2024 and regularly hinting he will run.

Bill Kilberg, an organiser of the Cheney fundraiser, told CBS: “I think people are really hungry for a sensible, rational alternative in our political dialogue. They’re not happy with the direction of the Republican party and they’re not particularly happy with the direction of the Democratic party.”

Referring to Cheney and Romney, he said attendees “saw two, sensible, intelligent, rational conservatives, and they were excited. It’s been a long time since we had that opportunity”.

Cheney has not ruled out a presidential run in 2024.

Romney was given standing ovations, Kilberg said, when he praised the Ukrainian president, Volodymyr Zelenskiy, and spoke about American democracy.

“He said we have to appreciate how fragile this system is.”

Kilberg’s wife, Bobbie, said: “That was the essence of Liz’s remarks as well. She said, if someone doesn’t have respect for the rule of law and the democratic system, then it’s all for naught.”