Clinton, a former first lady and U.S. senator, promised in her speech on Thursday to make the United States a country that works for everyone if she is elected.
"We are clear-eyed about what our country is up against. But we are not afraid," she said. Clinton, 68, portrayed Trump, 70, as a threat to the country, saying "a man you can bait with a tweet is not a man we can trust with nuclear weapons."
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 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.
The U.S. television audience for Clinton's acceptance speech appeared smaller than the viewership of Trump's address a week earlier, according to preliminary ratings data released on Friday.
An estimated 27.8 million people watched Clinton across six broadcast and cable networks, early Nielsen data showed. Trump had pulled in roughly 30 million from those networks. Updated figures were expected later on Friday.
Economic issues will be crucial as the White House campaign enters its final three-month stretch. The U.S. economy grew by only 1.2 percent in the second quarter, far less than expected, the Commerce Department said on Friday.
During the Rust Belt trip, Clinton will detail her pledge to raise wages and to create jobs by unveiling a major infrastructure package within the first 100 days of her presidency, and encouraging companies to invest in workers.
The start of the Democratic convention was overshadowed by the resignation of Democratic National Committee Chairwoman Debbie Wasserman Schultz who quit over leaked emails showing party officials favored Clinton over her primary rival Bernie Sanders, a U.S. senator from Vermont.
Cyber security experts and U.S. officials said on Monday there was evidence that Russia engineered the release of the emails in order to influence the election. The Kremlin has denied the accusations.
Another hack came to light on Thursday, when four people familiar with the matter told Reuters the FBI is investigating a cyber attack against the Democratic Congressional Campaign Committee, which raises money for Democrats running for the U.S. House of Representatives. The DCCC confirmed on Friday that it had been the target of a cyber security incident.