CLEVELAND — Rivals Hillary Clinton and Donald Trump sprinted out of the campaign blocks on Monday to begin their two-month dash to the US presidential election, descending on Ohio as ground zero of their 2016 battle.

The two candidates used the Labor Day holiday -- the traditional launch of the home stretch of the presidential campaign -- to push their arguments that they would be best for working class Americans.

Democrat Clinton maintains an edge over Republican flagbearer Trump in national polls, has dramatically deeper ground operations in swing states, and trounced Trump in August fundraising.

But Trump’s unorthodox White House bid, including his campaign’s apparent imperviousness to criticism about his harsh rhetoric, assures a tight contest for the next 64 days.

"I’m not taking anybody, anywhere for granted," Clinton told a crowd of more than 1,000 at a picnic in Cleveland.

Highlighting the intensity of the fight for battleground states like Ohio, Trump was already on the ground in Cleveland for his own campaign events when Clinton arrived, their planes parking about two football fields apart on the tarmac.

"I’m ready. I’m more than ready," she said of the intense, two-month battle ahead as she attempts to become the first female US commander in chief.

But after a few days of rest from campaigning, Clinton coughed her way through portions of her Cleveland remarks.

Suffering one of her worst coughing bouts of the race, she paused to sip water, her voice reduced to a crackling whisper at times.

That’s sure to fuel critics’ contentions that Clinton, 68, has serious health problems, although Clinton herself dismissed such "conspiracy theories" when asked about them, chalking up her coughing to seasonal allergies.

Clinton debuted her new campaign plane -- with the slogan "Stronger Together" emblazoned on the side -- and it hosted guests with whom she has been reluctant to travel until now: reporters.

Under extensive criticism from her rival and journalists for not holding a full press conference in nine months, Clinton welcomed reporters on her plane.

She answered questions for more than 22 minutes on several topics, including tensions with Russia over accusations of cyber-espionage.

Clinton expressed "grave" concern about reports that Russia has been interfering in the US electoral process through invasive cyber attacks on the Democratic Party and an apparent attack on voter registration systems in Arizona.

And she implied Moscow was trying to help get Trump elected.

"I think it’s quite intriguing that this activity has happened around the time Trump became the nominee," she said.

Republican messenger

Like Clinton, Trump has largely avoided having the press corps on his plane, but on Monday he invited some journalists aboard, where he discussed his immigration platform.

Just a week after traveling to Mexico and then returning across the US border to deliver a hardline immigration speech, he assailed Clinton for having "no plan" on immigration.

"What her real plan is, she has total amnesty" and a pathway to citizenship, he said, reiterating his opposition to such a legalization process without undocumented immigrants leaving the country first.

Under Clinton, "people can pour across the border and it doesn’t matter who the people are."

Clinton shot back by recalling Trump’s meeting with Mexico’s President Enrique Pena Nieto and their clash over Trump’s plan to have Mexico pay for a border wall.

Trump "can’t even go to a friendly foreign country without getting into a fight," she said during a campaign stop in Hampton, Illinois.

Trump, who visited a Cleveland diner to meet with union members, is seeking to capitalize on simmering frustration among blue-collar workers over jobs and wages.

"Voters who traditionally haven’t voted Republican or haven’t voted in a very long time seem to be coming out to support this messenger and this message," Trump’s campaign director Kellyanne Conway said.

Trump, 70, dominated last week’s political messaging and imagery that included his visit to an African-American church in Detroit.

And while Sunday’s CBS News Battleground Tracker shows Clinton, 68, leading Trump in two key states -- by eight points in Pennsylvania and four points in North Carolina -- recent polls show the race tightening nationally. — AFP