With two weeks to go until Election Day, President Donald Trump and Joe Biden adopted radically different strategies to secure votes -- the Republican incumbent ramped up his appearances and the Democratic challenger opted to hunker down at home.

After two campaign rallies on Monday in Arizona, the 74-year-old Trump heads Tuesday to Pennsylvania, making a new push in the battleground state seen as vital to his reelection bid -- although a rare appearance by first lady Melania Trump was called off at the last minute.

The former vice president, 77 -- who is leading Trump in the polls ahead of the November 3 showdown at the ballot box -- meanwhile did not schedule any public events for the second day running.

"Things are changing fast," Trump told the Fox & Friends program, claiming that his own polling showed Biden is "imploding."

"He's gone into hiding.... He's been there for a long time."

There is little visible evidence that the Democrat is really flagging in terms of voter support -- two days before the final televised debate with the Republican president, he appears to be preparing at home.

In fact, polls show Trump down or in a tight contest in most of the swing states that decide presidential races, but he believes a combination of frenetic campaigning and scorched earth personal attacks on Biden are giving him new momentum.

Trump went on the offensive as fiercely as ever Tuesday, flooding the zone with an intense pursuit of a murky story painting Biden as corrupt, calling on US Attorney General Bill Barr to investigate his "criminal" opponent.

However, just over two weeks ago Trump was hospitalized with Covid-19 and Biden had the field to himself, hammering home his central message that the Republican leader has failed the country on managing the coronavirus crisis.

Investigate 'before the election' 

Trump is the subject of multiple accusations of sexual assault, financial improprieties, and also is the first president to run for reelection after being impeached.

However, he won in 2016 in part thanks to the success of a last minute push to sow doubt about the honesty of his opponent Hillary Clinton.

This time, he has dusted off that playbook again.

His 2020 attack centers on a conspiracy theory about the Biden family's business activities. Crowds at Trump's rallies have even repurposed the old anti-Clinton chant for Biden, shouting "lock him up."

According to Trump, Biden's personally troubled son Hunter sold access to his father in Ukraine and China when he was vice president to Barack Obama.

The narrative has been given new life on the eve of the election with the release of a story in the Trump-friendly New York Post, based on information supplied by Trump allies, regarding the supposed discovery of incriminating information on a laptop belonging to Hunter Biden.

Trump took his tactics a step further Tuesday when he not only called Joe Biden "a criminal" but demanded on Fox News that Barr "act and act fast."

"This has to be known about before the election," Trump said.

Pennsylvania push 

According to a new poll from The New York Times/Siena College, Biden has a nine-point lead nationally. 

Although this reflects the consensus of polls, there are outliers, including IBD/TIPP, which accurately predicted the 2016 shock result and sees a much tighter race now, with Biden only 2.3 points ahead.

Pennsylvania is one of the half-dozen states that Trump more or less has to win to hold on to the presidency. Polls show Biden ahead, though a Reuters/Ipsos poll this week showed the gap narrowing slightly.

Trump's choice of Erie for Tuesday's rally matters because he won in the long-time Democratic bastion in 2016, highlighting his ability to woo the white, non-college educated, working class vote with his populist rhetoric.

Melania Trump was to have joined him in Pennsylvania -- her first campaign rally in more than a year and a potential boost with the women's vote where polls show Trump in serious trouble.

But the trip was canceled due to a "lingering cough" following her infection with the coronavirus, a spokeswoman said.

While Biden continues to try to focus on Trump's handling of the pandemic -- depicting him as an irresponsible leader who panicked -- the president is milking frustration in places like Pennsylvania with the economic impact of shutdowns and social distancing.

"We will never shut down" again, Trump promised Tuesday.

A key element in this year's campaign is early voting, which is at record highs amid fears about the spread of the virus. So far, some 34 million Americans have already cast ballots, according to the independent US Elections Project.