Setting the stage on Labour Day for a critical month in their testy presidential campaign, Donald Trump softened his stance on immigration while Hillary Clinton blasted Russia for its suspected tampering in the U.S. electoral process.

In a rare news conference aboard her new campaign plane, Clinton said she is concerned about "credible reports about Russian government interference in our elections."

"We are going to have to take those threats and attacks seriously," Clinton told reporters travelling with her from Ohio to Illinois.

Clinton's comments follow reports that the Russian government may have been involved in the hacking of Democratic National Committee emails just days before the party's national convention. The emails, later revealed by WikiLeaks, showed some DNC officials favouring Clinton over her primary opponent, Bernie Sanders — who has since endorsed Clinton for president.

Clinton speaks during a campaign stop at the 11th Congressional District Labour Day Parade and Festival in Cleveland, Ohio. (Brian Snyder/Reuters)

She said Russian President Vladimir Putin appears "quite satisfied with himself" and said Trump "has generally parroted what is a Putin-Kremlin line."

Trump won't rule out pathways to legal status

Meanwhile, Trump extended a rare invitation to journalists to accompany him on his private plane from Cleveland to Youngstown, Ohio. The billionaire businessman appeared to pivot away from his hard-line position on immigration, saying, "I'm all about jobs now."

Any immigrants who want full citizenship must return to their countries of origin and get in line, he told reporters — but he would not rule out a pathway to legal status for the millions living in the U.S. illegally, as he did in a long-awaited policy speech last week.

"We're going to make that decision into the future," Trump said.

Clinton powered through a coughing fit at a Labour Day festival at a Cleveland park, sharply criticizing Trump's recent trip to Mexico as "an embarrassing international incident."

Unwilling to allow Trump to modify his immigration stances, she said his address later that night in Arizona amounted to a "doubling down on his absurd plan to send a deportation force to round up 16 million people."

"He can try to fool voters into thinking somehow he's not as harsh and inhumane as he seems, but it's too late," Clinton said.

Trump greets supporters during a campaign stop at the Canfield County Fair in Canfield, Ohio. (Mike Segar/Reuters)

Clinton's 25-minute question-and-answer session was her first extensive availability with reporters since early December. Beyond Russia, she answered questions about the ongoing controversy surrounding her use of a private email server while secretary of state, which Trump has used to cast doubt over her ability to protect classified information.

"I take classification seriously," she said.

Trump told reporters Monday that "on occasion," he will invite journalists to travel with him.

'Just shows you how important Ohio is'

The two campaigns arrived in Cleveland within hours of each other, underscoring Ohio's quadrennial role in presidential campaigns. No Republican has won the White House without carrying the state, and the airport offered vivid imagery of that crucial role.

The airplanes of Trump and his running mate, Mike Pence, were parked on the tarmac as Clinton and her vice-presidential pick, Tim Kaine, arrived in separate planes. It was a near-encounter that even forced the Trump press corps to the side of the road as Clinton's motorcade whizzed by.

Trump's campaign plane, right, sits on the same tarmac as Clinton's as the two candidates campaign in Ohio on Labour Day. (Mike Segar/Reuters)

"It's kind of interesting to have all the planes here on the same tarmac," Kaine said after he and Clinton greeted supporters. "Just shows you how important Ohio is. We're going to be here a lot."

While Labour Day has traditionally been the kickoff to the fall campaign, both Clinton and Trump have been locked in an intense back-and-forth throughout the summer.

The start of full-fledged campaigning opens a pivotal month, culminating in the first presidential debate Sept. 26 at Hofstra University in Hempstead, New York. Polls show Trump trailing Clinton in a series of must-win battleground states, meaning the debates could be his best chance at reorienting the race.

Trump told reporters he does plan to take part in all three presidential debates, joking that only a "hurricane" or "natural disaster" would prevent him from attending.

Clinton will have millions of dollars at her disposal this fall to air television advertising and power a sophisticated get-out-the vote operation in key states.

The former secretary of state raised a combined $143 million in August for her campaign, the Democratic National Committee and state parties — her best month yet. She began September with more than $68 million in her campaign's bank account to use against Trump, who has not yet released initial fundraising totals for August.

Trump followed through with his vow to spend some $10 million on commercials in key states over the past week.