Donald Trump has accused rival Hillary Clinton of being a "world class liar" who personally profited from her tenure at the US State Department.
The presumptive Republican presidential nominee said of Mrs Clinton "she gets rich making you poor", during an address at his hotel in New York.
Seeking to steady his campaign after a difficult period, Mr Trump cast himself as the White House candidate best positioned to address Americans' economic interests.
"This election will decide whether we're ruled by the people or the politicians" he said.
He made his arguments in a pointed yet measured tone, less loud and strident than has been typical in most previous campaign speeches.
Yet his remarks included erroneous statements and distortions about Mrs Clinton's record, and he frequently referenced sources of information that have been widely questioned, including the book Clinton Cash by Peter Schweizer.
The speech marked an opening salvo against Mrs Clinton, the presumptive Democratic presidential nominee, in the general election campaign. It came as the billionaire faced growing questions about his readiness not just for the presidency, but for the campaign he will need to run to get there.
Mr Trump made a direct appeal to supporters of Mrs Clinton's primary rival Bernie Sanders, urging them to help him fix a "rigged system".
He also argued his trade and immigration policies would be more beneficial than Mrs Clinton's for black and Hispanic people, two groups that have overwhelmingly voted for Democrats in recent presidential elections.
Still, the central goal of Trump's speech appeared to be uniting the Republican Party and appealing to people who may be sceptical of him but vigorously oppose Mrs Clinton.
He also referred to Mrs Clinton's use of a private email server while she was secretary of state, her response to the deadly 2012 attacks on Americans in Benghazi, Libya, and accusations that she and former president Bill Clinton profited from their family foundation.
"Hillary Clinton may be the most corrupt person ever to seek the presidency," Mr Trump said.
Some of Mr Trump's charges come from Schweizer's book, which has been criticised for failing to provide evidence of direct connections between business dealings by foreign interests, sometimes involving the Clinton Foundation, and decisions by Mrs Clinton when she was secretary of state.
The Republican candidate also argued that Mrs Clinton's opposition to a massive Asia-Pacific trade pact was an example of her political opportunism, noting that she had worked on the agreement while at the State Department and has previously backed free trade accords.
And he took aim at her immigration policies, saying her call for providing those in the US illegally with a path to citizenship amounted to "mass amnesty".
Mr Trump's remarks came one day after Mrs Clinton launched her own blistering attacks on her White House rival. She moved to undercut Mr Trump's argument that his business record would help him create jobs as president, arguing instead that he had been "reckless" with his companies and "shouldn't have his hands on our economy".
Mr Trump also tried to turn Mrs Clinton's strengths into negatives. He spent much of his speech trying to undercut her foreign policy record, highlighting her early support of the 2003 invasion of Iraq, which she now says was a mistake, and linking the policies she promoted at the State Department to the rise of the Islamic State terror group.
"The Hillary Clinton foreign policy has cost America thousands of lives and trillions of dollars and trillions of dollars and unleashed ISIS across the world," Mr Trump said.
Mr Trump's speech came on the heels of his firing on Monday of campaign manager Corey Lewandowski, a controversial figure who was seen as an impediment to efforts to build a more robust campaign organisation.
A new fundraising report released hours after Mr Lewandowski's firing showed how much ground Mr Trump has to make up. He started June with just $1.3m (€1.15m) in the bank.
Mr Trump's allies cast Mr Lewandowski's firing this week as the start of a new phase for the campaign.
Paul Manafort, the campaign chairman and Mr Lewandowski's internal rival, signalled on a conference call with aides that a rapid staffing expansion would be coming soon.
Tennessee Republican Senator Bob Corker, who has been seen as a potential vice presidential pick, said he was "pretty excited" to learn of the changes.
"I think that what appears to be occurring over the last 24 hours is a movement in a direction that I think could be very, very positive," Mr Corker said.