As Donald Trump opened fire on Justin Amash, the Michigan representative who became the first Republican in Congress to say he had engaged in “impeachable conduct”, Mitt Romney declined to join the fight.

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The former presidential nominee and Republican senator from Utah accused Trump of lacking humility, honesty and integrity – but stopped short of calling for his removal from power.

Romney was scathing about the picture of the president that emerges from the Mueller report into Russian interference in the 2016 election, the redacted version of which he said he had spent two days reading in full. He said on Sunday its findings were “troubling, unfortunate and distressing”.

But he said he did not think it was time for Congress to call for impeachment.

“I don’t think there is the full element which you need to prove the obstruction of justice case,” he told CNN’s State of the Union.

Mueller did not find that Trump or his aides conspired with Russia but he did lay out 11 instances of possible obstruction of justice by Trump or his campaign and indicated Congress should decide how to proceed. Controversially, attorney general William Barr said in his own summary of the then unseen Mueller report that Trump had not obstructed justice.

I don’t think there is the full element which you need to prove the obstruction of justice case Mitt Romney

Romney’s sharp but qualified criticism of Trump came a day after Amash became the first Republican to break ranks and call for impeachment. In a stream of tweets, Amash said the Mueller report showed “President Trump engaged in specific actions and a pattern of behavior that meet the threshold for impeachment”.

Amash and Romney are significant figures within their party, as they stand virtually alone in having the temerity to challenge Trump in public. But the fact that Romney would not join Amash on impeachment is an indication of the impenetrable wall of opposition the party is likely to erect should Democrats initiate proceedings.

Just why became clear later on Sunday, when Trump aimed his Twitter account at Amash.

Saying he was “never a fan”, he called Amash “a total lightweight who opposes me and some of our great Republican ideas and policies just for the sake of getting his name out there through controversy”.

Trump also accused Amash of not having read the Mueller report – the congressman made much of saying he had in fact read all 448 pages – and, while repeating familiar complaints about Mueller, wrote: “Justin is a loser who sadly plays right into our opponents [sic] hands!”

In fact, Amash’s sole call for impeachment on the Republican side may not do much to move the political dial. Democrats are edging closer to launching proceedings, but not for the reasons the congressman outlined.

Adam Schiff, the Democratic chair of the House intelligence committee, told CBS’s Face the Nation there were no signs of the Republican-controlled Senate moving towards impeachment. Nonetheless, Democrats were becoming more minded to take it on, he said, as a tool to increase pressure on the Trump administration to hand over key documents, including the unredacted Mueller report, that it is refusing to submit to congressional oversight.

Facebook Twitter Pinterest Donald Trump and Mitt Romney dine in New York City, in November 2016. Photograph: Drew Angerer/Getty Images

“What may be pushing towards impeachment has less to do with Justin Amash and more to do with the administration engaging in a maximum obsctructionism campaign against Congress,” Schiff said.

Romney, who ran unsuccessfully against Barack Obama in 2012, said impeachment was not just a legal matter. It must also, he said, “consider the practicality of politics, and the American people are just not there”.

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He added: “The Senate is not there either.”

Democratic leadership has also considered public opinion, and what impeachment might do to motivate Trump’s base, when weighing up whether to make the move.

Despite his reluctance to go all the way into impeachment, Romney has showed himself willing to take on Trump. In April he issued a statement saying: “I am sickened at the extent and pervasiveness of dishonesty and misdirection by individuals in the highest office of the land, including the president.”

He told CNN the Mueller report distressed him.

“The number of times there were items of dishonesty, misleading the American public and media – those are not things you would want to see from the highest office in the land.”

He said that in terms of three crucial features of a president – humility, honesty and integrity – Trump has “distanced himself from some of the best qualities of the human character”.

Such was his disgust with Trump as a presidential candidate in 2016, Romney wrote in his wife Ann on to the presidential election ballot, thereby voting for her instead. He told CNN he had not yet decided if he would do the same next year.