Democrat Hillary Clinton is taking her freshly energised campaign to become America's first woman president to the 'Rust Belt' states that might decide the fate of the November 8 election.
After presenting an upbeat view of the country in her keynote address to the Democratic convention on Thursday night, the former secretary of state launched a campaign tour of Ohio and Pennsylvania, two heartland states hit by the decline in US manufacturing.
Ms Clinton is likely to face a tough challenge in such states from Republican nominee Donald Trump, a New York businessman who is trying to win white working-class voters with rhetoric against free trade and illegal immigration.
"There is no doubt in my mind that every election in our democracy is important in its own way but I can't think of an election that is more important, certainly in my lifetime," Ms Clinton told a rally in Philadelphia on Friday (local time).
"And it's not so much that I'm on the ticket it's because of the stark choice that's posed to America in this election," she added.
In the biggest speech of her quarter century in politics, Ms Clinton formally accepted the Democratic presidential nomination on Thursday at the convention in Philadelphia.
Ms Clinton and her vice presidential running mate, Senator Tim Kaine of Virginia, addressed a crowd of more than 5000 people at Temple University near downtown Philadelphia, before heading out on their three-day bus tour.
Ms Clinton reprised themes from her Thursday night speech.
Opinion polls show a potentially tight race in Ohio and Pennsylvania, both of which were won by President Barack Obama in the 2012 election.
Ms Clinton and Mr Trump are essentially tied in Ohio, where the Republicans held their convention last week, according to an average of polls by RealClearPolitics.
Clinton has a lead of 4.4 percentage points in Pennsylvania, the website's average of recent polls showed.
Ohio and to a lesser extent Pennsylvania are among a handful of competitive states that are traditionally viewed as decisive in presidential elections, since they do not lean heavily either Democratic or Republican.
Nationally, opinion polls show Mr Trump moving into a slight lead after receiving his party's nomination at the convention in Cleveland.
Ms Clinton is likely to get a similar boost after the Democratic convention, where she was lauded by Mr Obama and other senior Democrats as a tough fighter with a long-held passion for helping the underprivileged.
Following Ms Clinton's speech, Mr Trump sent out a flurry of comments on Twitter on Friday morning, lambasting media coverage of the speech as "a joke," calling the address "very long and very boring" and accusing Ms Clinton of wanting to shut down "coal mines, steel plants and any other remaining manufacturing".
He will campaign in another swing state, Colorado, on Friday and is scheduled to visit Ohio next week.