Donald Trump is returning to the inflammatory playbook that helped lift him to victory nearly two years ago, waging a campaign of fear and racial division in an effort to save his party’s majorities in Congress.

With two weeks left before the midterm elections, Trump is amplifying a dark vision of what the country could look like if Democrats gain control in Congress. At campaign rallies, on Twitter and in his public commentary, he has issued urgent – if groundless – warnings about terrorists at the border, a socialist takeover and rampant crime.

Long on rhetoric but short on truth, the president has trained his focus on a migrant group trekking through Central America toward the US southern border, a procession the president has called “an assault on our country”. The so-called caravan has swelled to more than 7,000 Central Americans, many of whom have said they are fleeing poverty and violence in their native countries and dream of a better life in Mexico or the US.

Yet without citing evidence, Trump has claimed the caravan is embedded with criminals, gang members and “unknown Middle Easterners”, an apparent attempt to exploit fear of terrorism and immigration.

Are Donald Trump's claims about the caravan of 7,000 migrants accurate? Read more

“That is an assault on our country and in that caravan you have some very bad people and we can’t let that happen to our country,” Trump said at a rally in Houston on Monday night. He repeated a baseless claim that Democrats “had something to do with” organizing the migrant expedition. And then declared himself a “nationalist”, a term he typically avoids using directly about himself at such events. But in a sign that he wants to escalate the rhetoric, he shouted “use that word” to the cheering crowd. They erupted into chants of “U-S-A”.

After the White House press secretary, Sarah Sanders, said Trump “absolutely” had evidence that “Middle Easterners” were among the thousands of families making their way north – and after Vice-President Mike Pence supported the assertion –Trump admitted he had no proof to support his claim.

Speaking on Tuesday afternoon in the Oval Office, Trump said that “there’s no proof of anything” but added that there “very well could be” Middle Easterners among the families.

The racially charged language and a hardline stance on immigration were critical to his success in 2016, according to the political scientists John Sides, Michael Tesler and Lynn Vavreck in their new book, Identity Crisis, based on an analysis of polls, voting data and voter attitudinal surveys.

“The evidence is clear,” the authors write. “Both in the Republican primaries and in the general election, white voters’ attitudes about African Americans, Muslims and immigration were more closely associated with how they voted than were any strictly economic concerns”.

Facebook Twitter Pinterest Supporters cheer as Donald Trump speaks during a campaign rally at the Toyota Center in Houston, Texas, 22 October 2018. Photograph: Saul Loeb/AFP/Getty Images

Jeff Flake, the retiring Arizona senator and a Republican critic of Trump, said he was unnerved by the president’s use of divisive and racially inflammatory rhetoric. He called the insinuation that terrorists are traveling with the caravan a “canard” and a “fear tactic”.

“These, for the most part overwhelmingly, are people who are either fleeing violence or looking for a better life,” the senator said during a CNN forum in New York.

But his view stands in sharp contrast to attempts to stoke racial anxiety among white voters by dozens of Republican candidates running for Congress and the political action committees supporting them.

In a closely watched race in New York’s Catskills region, advertisements supporting the Republican incumbent John Faso have focused on his opponent’s brief rap career more than a decade ago, rather than his record at Harvard and as a Rhodes scholar. One ad brands the Democrat Antonio Delgado, who is black, a “big city rapper”. Delgado, a local attorney born in Syracuse, advocates for universal healthcare.

In this election’s first gubernatorial debate in Florida on Sunday, Andrew Gillum, the Democratic nominee who would be the state’s first African American governor, accused his opponent, Ron DeSantis, of attempting “to draw all the attention he can to the color of my skin”.

Republican attack ads echo Trump's anti-immigration message to whip up fear among supporters Read more

And in California, the congressman Duncan Hunter, a Republican who was indicted on charges he misused campaign funds, called his Democratic opponent, Ammar Campa-Najjar, who is Arab American, a “security risk” to the country. In an ad by the same name, Hunter accuses Campa-Najjar of trying to “infiltrate” the US government and accuses him of being supported by the Egyptian Islamist organization the Muslim Brotherhood.

Hunter’s ad is part of a trend this cycle, according to pre-election report titled Running on Hate that was commissioned by the national legal advocacy organization Muslim Advocates and released this month.

“Trump’s candidacy, election, and presidency emboldened a new wave of anti-Muslim conspiracy theorists to run for office nationwide and at all levels of government,” it said.

At the rally in Houston, Trump unleashed a torrent of immigration-related grievances in remarks riddled with falsehoods and conspiracies.

The increasingly inflammatory speech is matched by incidents of open racial bias in public that are being caught on camera.