A close United States Senate ally to Republican President Donald Trump warned on Thursday that Democrats have a “good chance” at winning the White House, as another coronavirus diagnosis rocked the election campaign 19 days out.

As Trump and rival Joe Biden prepared for separate, simultaneous town halls later on Thursday, Senator Lindsey Graham acknowledged out loud what major polls have been showing for months.

“You all have a good chance of winning the White House,” Graham, chairman of the Senate Judiciary Committee, told Democratic colleagues at the opening of a hearing on Trump's Supreme Court nominee Amy Coney Barrett.

The concession will buoy Biden as he and Trump — whose re-election bid has been floundering — hold their split-screen spectacle instead of clashing face-to-face in a second debate that would have involved answering questions from voters.

The plan changed when Trump contracted Covid-19 and then backed out of a virtual head-to-head, saying it would be in person or nothing for him.

The coronavirus pandemic, which has killed 217,000 Americans, will loom over both town halls, as Biden's vice presidential running mate Kamala Harris froze her travel after her communications director tested positive for Covid-19.

Nearly two weeks after the president himself contracted the virus, Harris staffer Liz Allen and a flight crew member were diagnosed late on Wednesday, Biden campaign manager Jen O'Malley Dillon said in a statement.

Harris did not need to quarantine, O'Malley Dillon added, but “out of an abundance of caution” would cancel travel and campaign virtually before hitting the road again October 19.

Both individuals flew with Harris on October 8. The campaign said they wore masks and were not within six feet of Harris for more than 15 minutes.

Two negative tests

“I wasn't in close contact — as defined by the CDC — with either during the two days prior to their positive tests,” tweeted Harris, a 55-year-old US senator.

“I've had two negative tests this week and am not showing symptoms.”

Biden has reported multiple negative coronavirus tests since Trump's Covid-19 diagnosis on October 1 but the campaign has not said whether he has been tested since the Harris staffer tested positive.

Doctors gave Trump the green light to head back onto the campaign trail at the weekend without providing proof of a negative PCR test — the industry standard for giving patients the all-clear — instead relying on less accurate rapid tests and other health metrics.

He has returned with a vengeance to the campaign trail, holding four mass rallies in four days this week, including a North Carolina gathering midday on Thursday.

But even as Trump sought to mount a closing argument against Biden at a boisterous rally on Wednesday in Iowa, promising an “incredible” third quarter for the struggling economy, the pandemic remained front and centre.

Trump's teenage son Barron contracted the virus, First Lady Melania Trump revealed in news that startled in part because it had been kept from the public.

She said the 14-year-old did not experience symptoms and has since tested negative.

“He had it such a short period of time I don't even think he knew he had it,” the president said in Des Moines.

While team Trump might be relieved that he regains an opportunity to draw a contrast with his opponent, NBC faced criticism for placing Trump in the same 8pm time slot as Biden.

'Unpredictability'

“Having dueling town halls is bad for democracy,” tweeted Katie Couric, a longtime host of NBC's “Today” show.

“Voters should be able to watch both and I don't think many will,” she said, adding that the matchup would benefit Trump "because people like to watch his unpredictability". David Canon, chair of the political science department at the University of Wisconsin-Madison, said Trump may have committed a “tactical error” by backing out of a virtual debate.

“He needed the debates more than Biden did,” Canon told AFP. “He's the one that needs to change the momentum in the election.”

Trump negotiated to go on NBC at an outdoor setting in Miami, where the audience will wear masks, after Biden arranged his own event on ABC in Philadelphia.

In Iowa Trump, 74, renewed his long-running narrative that 77-year-old Biden is too frail for the presidency.

“Joe is shot, OK?” Trump said.

“If he wins, the radical left will be running the country. They're addicted to power, and God help us if they get it.”

A RealClearPolitics poll average has Biden up by 9.2 points nationally and leading in several key battleground states.

A new NBC News/Wall Street Journal poll on Thursday has Biden ahead by 11 points nationally.