With Labour Day behind them, Donald Trump and Hillary Clinton are pushing ahead in top presidential battlegrounds in the South.
Mr Trump, the Republican nominee, is set to campaign in Virginia and North Carolina on Tuesday, two critical states in his path to the presidency.
Mrs Clinton, the Democrat candidate, is campaigning in Florida in search of an advantage in the nation's largest swing state. A Clinton victory in Florida would make it virtually impossible for Mr Trump to overcome her advantage in the race for 270 electoral votes.
On Monday, in swing state Ohio, Mr Trump softened his stance on immigration while Mrs Clinton blasted Russia for suspected tampering in the US electoral process.
In a rare news conference on board her new campaign plane, Mr 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," she told reporters travelling with her from Ohio to Illinois.
Mrs 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 Mrs Clinton over her primary opponent, Bernie Sanders - who has since endorsed Mrs Clinton for president.
She said Russian President Vladimir Putin appears "quite satisfied with himself" and said Mr Trump "has generally parroted what is a Putin-Kremlin line".
Meanwhile, Mr Trump extended a rare invitation to journalists to accompany him on his private plane from Cleveland to Youngstown, Ohio. The billionaire businessman appeared to shy away from his hard-line vow to block "amnesty" for immigrants in the country illegally.
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 US illegally, as he did in a long-awaited policy speech last week.
"We're going to make that decision into the future," said Mr Trump.
Mrs Clinton powered through a coughing fit at a Labour Day festival in a Cleveland park, sharply criticising Mr Trump's recent trip to Mexico as "an embarrassing international incident".
Unwilling to allow Mr 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," said Mrs Clinton.
The former secretary of state flatly said "No" when asked in an ABC News interview whether she would be willing to accept the Mexican president's invitation to visit the country, as Mr Trump did last week.
"I'm going to continue to focus on what we're doing to create jobs here at home," she said.
Earlier in the day, Mr Trump attacked Mrs Clinton's energy level, noting that she has not followed his aggressive travelling schedule and questioning whether she had the stamina to help bring jobs back to America.
"She doesn't have the energy to bring 'em back. You need energy, man," Mr Trump told reporters.
He added, "She didn't have the energy to go to Louisiana. And she didn't have the energy to go to Mexico."
Mrs 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 Mr Trump has used to cast doubt over her ability to protect classified information.
"I take classification seriously," she said.
While Labour Day has traditionally been the kick-off to the autumn campaign, both Mrs Clinton and Mr 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 at Hofstra University in Hempstead, New York,. on September 26. Polls show Mr Trump trailing Mrs Clinton in a series of must-win battleground states, meaning the debates could be his best chance at reorienting the race.
Mr 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.