After boasting about the economy and raising fears over immigration, US President Donald Trump is facing pushback from his predecessor Barack Obama, who is taking on an increasingly prominent role in the final weekend of campaigning before midterm elections in which Republican control of Congress is threatened.

With rallies planned in Montana and Florida, a state he had already visited on Wednesday, Trump on Saturday is keeping up his relentless campaign schedule before Tuesday's ballot, which has become a referendum on his unconventional presidency.

"Heading to Montana and Florida today! Everyone is excited about the Jobs Numbers - 250,000 new jobs in October. Also, wages rising. Wow!" Trump said on Twitter Saturday morning.

The campaigning comes one week after a gunman, who allegedly hated immigrants and Jews, killed 11 people at a synagogue in Pittsburgh, and after a fanatical Trump supporter was arrested in Florida on charges of mailing homemade bombs to more than a dozen Trump opponents, including Obama.

At a moment of deep national division, with the political temperature soaring, the president's critics say he has helped create an atmosphere in which the two attackers felt comfortable to carry out their crimes.

Trump says his Republicans are in a good position ahead of the midterm congressionalelections, particularly with new employment figures out showing the economy booming.

But polls point to the Democrats capturing at least the House of Representatives, threatening the billionaire president with the specter of an opposition finally able to block policies and dig into his highly opaque personal finances.

In the last stages of the campaign, Trump is dueling with former president Obama, who came out of relative seclusion to appear at a Florida rally on Friday.

Obama is to campaign again Sunday in his hometown of Chicago, as well as in Indiana, where the seat of Democratic Senator Joe Donnelly is in danger.

- America at 'a crossroads' -

At a rally on Friday, Trump called America's first black president "Barack H. Obama," a reference to Obama's middle name of Hussein. Before his presidential run, Trump fanned conspiracy theories about the origins of Obama, who was born in Hawaii. (AFP)