Barack Obama versus Donald Trump. It is the presidential election that never happened but has come to define an era of US politics.

Obama is back on the campaign trail for the midterms, still electrifying crowds, earning shouts of “We love you!” and skewering his successor. “What’s up with the gold?” he asked a crowd in Detroit. “I mean, he’s got the White House looking like Saddam Hussein opened a Cheesecake Factory.” This has, predictably, set off a mini culture war involving questions of elitism and the tastefulness of chain restaurants.

Trump’s election to the White House 10 years ago next month owed much to a racist backlash against America’s first Black president as white Christians shifted from majority to minority. He had pushed conspiracy theories about Obama’s birthplace and promised to build a border wall to keep out criminals and rapists.

Indeed, election campaigns are a furnace in which pretence burns away and essential character is exposed. They certainly bring out the worst in Trump. This year’s midterms effort, in which he is averaging about a rally a day and fighting bad polls like a cornered animal, is a case in point.

Has any president’s closing message ever been so Islamophobic, misogynistic and racist, not to mention downright deranged and bizarre? It is like watching a man vomit up bigotry that has been poisoning his soul for decades. Just when you think Trump has hit rock bottom, he produces a jackhammer and starts tunnelling to a new nadir of nastiness.

He still relishes calling Obama by his full name, Barack Hussein Obama, with an emphases on Hussein. Now he has Democratic candidate Abdul El-Sayed, who would become the first Muslim senator if elected, in his sights.

In Port Huron, Michigan, on Sunday, he called El-Sayed a “jihadist” who is “on the side of the terrorists” and would “have to go through a metal detector before he’s allowed to go into the Senate”. Trump repeatedly invoked the 11 September 2001 terrorist attacks, drawing a comparison between stopping El-Sayed from entering the Senate chamber and the passengers who fought back against the hijackers onboard United flight 93.

There is no evidence for any of this. But Trump is on an Islamophobic tear. He has repeatedly attacked Ilhan Omar, the Somali-born congresswoman of Minnesota, while his justice department banged on her door at 6am to investigate alleged fraud in her family’s immigration paperwork. Omar has said she is “confident in the fact that I didn’t do anything wrong”.

One of Trump’s standard punchlines now is the racist and false assertion that 98% of people from Somalia in the US don’t work for a living. “They don’t really have to work because they steal so much,” he told one rally. “They all have a Mercedes-Benz.”

He told his overwhelmingly white fanbase: “Can you believe, low IQ, and they live better than you do? I don’t know what’s going on. Maybe they’re not as low as we think.”

Another highly peculiar theme of Trump’s rally blitz has been his obsession with the male physique. He said of the Ohio senator Jon Husted: “I looked at his ass, and then I looked at his legs. I said, ‘His ass, shit, it’s like, it’s like a pile of muscle.’” He also claimed to have “fallen in love” with Tim Scott, the South Carolina senator.

The objectification is not limited to men. Back in Michigan, Trump turned his attention to the posterior of the rapper Nicki Minaj. “I looked at one hell of a beautiful ass before: Nicki Minaj,” he said. “That’s hard to miss. Stand up! That’s hard to miss.”

How is all of this resonating with Trump’s supporters? I called Sharon Anderson, who has been to 81 Trump rallies and was entirely untroubled by his comment. She said: “If a woman puts her ass out there for attention and somebody pays attention to it – Trump didn’t try to rape her – what’s the big deal?”

Anderson, 70, who is from Etowah, Tennessee, and went to her first Trump rally in 2015, dismisses the notion that he is getting worse. “I’ve been doing this for 11 years. He’s not changed a bit in my opinion. Now, a lot of what he says is reported and a lot of what he says isn’t reported.”

Does she have any concerns about the president’s mental health? “Absolutely not! Do you know anyone that could do what he does? He speaks an hour and a half to an hour and 45 minutes every rally. It’s almost supernatural in my opinion,” she said.

Yet Trump, who is 80, has started making strange guttural sounds, somewhere between a burp and a hiccup, in the middle of a sentence. And on Sunday he unleashed a 1,300-word rant on social media about his relationship with the comedian Bill Maher, branding him a “COMPLETE AND TOTAL LOSER” and telling him: “Bill: Don’t call anymore. It’s useless.”

Judging by the midterm polls that favour Democrats, Obama might have the upper hand in this latest struggle between the two men. As power slips inexorably from Trump’s grasp, as it must for all lame duck presidents, how much more desperate and extreme can he become? What new degradations await?

His rantings and ravings are a grim omen as he enters the Apocalypse Now phase of his presidency. Like Col Kurtz, he is a god to a fanatical but shrinking band of followers. Like Col Kurtz, he may be on a final descent into madness. Two more years? The horror, the horror.