WASHINGTON - President Donald Trump, clearly concerned at trailing presumptive opponent Joe Biden in polls, is plunging back into the campaign for the November 3 election, pressing hot buttons to invigorate his base.

In his comeback rally on Saturday in Tulsa, Oklahoma - a comfortably Republican state - the President stressed law and order, promised economic recovery and slammed China for the coronavirus pandemic.

Mr Trump is due to visit a section of newly constructed border wall - promised on the 2016 campaign trail - in Arizona on Tuesday.

But the Tulsa rally was a stuttering return. It drew a smaller-than-expected crowd and the 19,000-seat indoor remained half empty. Several of the President's advance team tested positive for Covid-19. Many rally attendees did not wear the masks provided,since wearing them was not mandatory.

And in the course of adlibbing during a speech that lasted almost two hours, the president raised eyebrows when he said he had asked for coronavirus testing to be slowed because it was revealing more cases.

"When you do testing to that extent, you're gonna find more people, you're gonna find more cases," he said.

"So I said to my people, slow the testing down, please."

A White House official later said the remark was meant to be a joke.

Mr Trump also blamed "radical fake news" media for not crediting him with doing "a phenomenal job" responding to the pandemic. And he slammed China, at one point referring to the Covid-19 virus as "kung flu" - widely considered an offensive term.

He also railed against the movement to remove statues of prominent Confederate figures from public places, which was sparked by the death of African American George Floyd at the hands of a white police officer and the nationwide protests against racism and police brutality that followed.

In America's 1861 to 1865 civil war, the Confederacy fought to secede and maintain slavery. Statues of Confederate heroes dot the United States, especially in the south, where Confederate flags and bumper stickers are also not uncommon.

President Trump slammed "the unhinged left wing mob… trying to vandalise our history."

"They are trying to destroy our heritage, now they're tearing down our statues, our beautiful monuments unless we conform to their views," he said. "Well, we aren't conforming and that's why we're here."

The attacks come as Joe Biden, former vice president and presumptive Democratic Party candidate, maintains a steady and even widening lead over President Trump.

While several states emerge from lockdowns - and deal with spiking Covid-19 cases - Mr Biden has yet to hold large scale rallies.

A Fox News poll released on Thursday found that 50 per cent of respondents would vote for Mr Biden, compared to 38 per cent for Mr Trump - a wider gap than in a previous poll in mid May.

The President's tough talk and harsh police response to the protests following the death of George Floyd may have contributed to the widening gap.

In the Fox survey, 61 per cent disapproved of his handling of race relations, with only 32 per cent per cent approving.

Separately, a Quinnipiac University poll also released on Thursday found Mr Biden leading Mr Trump by 8 points.

A June 18 summary by pollster Nate Silver of the Five Thirty Eight website showed Mr Biden with a solid 9.2 point lead over Mr Trump nationally. But the gap in key swing states is narrower.

"Biden also leads Trump in swing states such as Michigan, Wisconsin and Arizona - although his lead in many swing states are not as wide as his margin in national polls, suggesting that the Electoral College could once again favour Trump in the event of a close election," Mr Silver wrote.

President Trump has shrugged off the polls. "I stand before you to declare that the silent majority is strong as ever," he told the crowd at the Tulsa rally.

Reflecting the increasing edginess of the national mood amid the pandemic, the induced economic crash and protests over racism, security at Tulsa was particularly tight, with armed National Guard troops deployed.

But while there were protesters at the event, there was no violence.

And pundits are unwilling to call the race for the White House so early. In his June 18 assessment Mr Silver wrote : "While a Biden landslide is possible if he wins all these swing states, so is a Trump Electoral College victory, depending on which way the race moves between now and November."