Former President Barack Obama’s voice has a way of lifting into a high-pitched tone of astonishment when he talks about his successor, almost as if he still cannot believe that the Executive Mansion he occupied for eight years is now the home of President Donald Trump.

For most of the last two years, he stewed about it in private, only occasionally speaking out. But as he hit the campaign trail this fall, Obama has vented his exasperation loud and often, assailing his successor in a sharper, more systematic way arguably than any former President has done in three-quarters of a century.

Although some admirers believe he remains too restrained in an era of Trumpian bombast, Obama has excoriated the incumbent for “lying” and “fear-mongering” and pulling “a political stunt” by sending troops to the border.

As he opened a final weekend of campaigning before Tuesday’s midterm elections, Obama has re-emerged as the Democrats’ most prominent face, pitting President versus President over the future of the country.

In a fiery speech in Miami on Friday afternoon before heading to Georgia for another rally, Obama said that even conservatives should be disturbed by Trump’s disregard for the Constitution and basic decency.

“I know there are sincere conservatives who are compassionate and must think there is nothing compassionate about ripping immigrant children from the arms of their mothers at the border,” he said.

“I am assuming that they recognise that a President doesn’t get to decide on his own who’s an American citizen and who’s not,” he continued, referring to Trump’s vow to sign an executive order cancelling birthright citizenship for the children of illegal immigrants.

“That’s not how the Constitution of the United States works. That’s not how the Bill of Rights works. That’s not how our democracy works.”

“I’m assuming people must get upset,” he went on, “when they see folks who spend all their time vilifying others, questioning their patriotism, calling them enemies of the people and then suddenly pretending they’re concerned about civility.”

The current President fired back later in the afternoon.

Trump, who has made more than 6,400 false or misleading statements since taking office, according to a count by The Washington Post, said his predecessor had lied by telling Americans they could keep their doctor under his health-care plan, which ultimately turned out not to be the case.

“Twenty-eight times he said you can keep your doctor if you like your doctor,” he told a small crowd at a West Virginia airport hangar. “They were all lies. Used it to pass a terrible health-care plan we are decimating strike by strike.”

Since leaving office, Obama has risen in the esteem of many Americans, as former Presidents often do. A poll by CNN this year found that 66 per cent had a favourable view of him, far more than those who approve of Trump’s performance in office.

When he left the White House in January 2017, Obama said he intended to follow the tradition of his predecessors by staying out of the spotlight unless he perceived what he considered broader threats to American values.

Advisers said Trump’s performance in office had qualified, justifying his decision to abandon restraint this fall.

“He cares very deeply,” said Valerie Jarrett, his longtime friend and adviser. “His language has been very direct and he’s made an appeal to citizens across our country that now’s the time to stand up for our core ideals.”

He has issued 350 endorsements that candidates then trumpeted on social media and he has helped raise millions of dollars for Democrats.

A video op-ed he taped generated 17 million views and a voter registration video drove nearly 700,000 viewers to Vote.org, according to his team. He is taping dozens of recorded telephone messages that will be sent out this weekend.

Obama’s red-meat speech on Friday delighted the crowd at the Ice Palace Film Studios in Miami. But if he has become the Democrats’ “forever President”, as Andrew Gillum, the party’s candidate for governor of Florida, called him, there are trade-offs for an Opposition party trying to groom a new generation of leaders as the start of the 2020 presidential election approaches.

“President Obama wants to make room for the next generation of Democratic leaders to step up, which is why he’s largely stayed out of the day-to-day fray over the past two years,” said Eric Schultz, a senior adviser to the former President. “But too much is at stake in these midterms and this moment is too consequential to sit out.”

To Republicans, Obama’s decision to directly take on his successor smacks of violating norms just as he accuses Trump of doing.

Not since Theodore Roosevelt and Herbert Hoover has a President hit the campaign trail after leaving office to actively take on his successor in quite the way Obama has.

Roosevelt actually mounted a comeback against his handpicked replacement, Wiliam Howard Taft, while Hoover castigated Franklin D. Roosevelt’s New Deal programme as “despotism” at the Republican convention in 1936.

Other former Presidents have been critical of their successors, too. Jimmy Carter became a vocal opponent of George W. Bush’s invasion of Iraq, calling his administration the “worst in history”.

But with Carter and others, these were one-off comments in interviews or other public settings, not a systematic indictment on the campaign trail.