Joe Biden is capitalising on having the campaign trail largely to himself and expanding his lead in the US polls. 

The Democratic presidential nominee is enjoying a boost in the polls following Donald Trump's Covid-19 diagnosis and the first presidential debate. 

A look at the seven-day rolling average of the approval rating of US Presidential candidates Joe Biden and Donald Trump, courtesy of RealClearPolitics.

Biden has been hitting critical swing states and investing in long-time Republican bastions that he hopes might expand his path to victory as the campaign heats up in the build-up to November's election. 

The Democratic presidential nominee made his second trip to Florida in a little over two weeks on Monday while his opponent President Donald Trump continues his recovery from coronavirus.

His visit to Miami was designed to encroach on some of Mr Trump's turf, even in Little Havana, a typically conservative area known for its staunch opposition to the communist government that Fidel Castro installed in Cuba.

He will follow up with a trip later this week to Arizona, which has not backed a Democratic presidential candidate since 1996.

Even Mr Biden's former primary rival, Bernie Sanders, has resumed in-person campaigning for the first time since the coronavirus outbreak in March.

The progressive Vermont senator held socially distanced rallies in the battlegrounds of New Hampshire and Michigan, proclaiming: "We need Joe Biden as our president."

Joe Biden is targeting spending to put Donald Trump on defence across the country and build an advantage in the Electoral College so large that the president might struggle to contest it.

READ MORE: Donald Trump returns to White House after hospital admission for Covid-19

Trump, who lost the popular vote in 2016, has said he may not accept the election results this year and has raised unfounded allegations that the increased use of mail-in voting amid the coronavirus pandemic could lead to fraud.

Mr Biden is complementing the expanded campaign travel with a late-stage ad push, reserving more than six million US dollars in television airtime in Texas, for decades deeply red, through the end of October, according to an Associated Press analysis of CMAG data.

He also plans to spend four million US dollars on advertising in Georgia, another Republican-leaning state that Democrats are feeling bullish about.

Mr Trump, meanwhile, has scaled back advertising in both states and has begun doing the same in Ohio, which he also won in 2016.

Staging a dramatic return to the White House after leaving the military hospital following his Covid-19 diagnosis, Mr Trump gingerly climbed the South Portico steps, removed his mask and declared: "I feel good."

READ MORE: Presidential debate recap: Trump and Biden's furious clash

He also hailed his actions saying: "I stood out front. I led. Nobody that's a leader would not do what I did. I know there's a risk, there's a danger, but that's OK.

"And now I am better, maybe I'm immune. I don't know"

Mr Trump's message alarmed infectious disease experts and suggested the president's own illness had not caused him to rethink his often-cavalier attitude toward the disease, which many experts say is being reflected in the polls. 

Democratic strategist Brad Bannon said: "It would be a mistake for Biden to take his foot off the pedal", but added: "I wouldn't want Biden to make the same mistake Clinton did."

"I'd be careful, if I was Biden, about expanding the map too much," Mr Bannon said.