The Brazilian Air Force reported the plane was a Cessna 560XL.
The plane carrying Eduardo Campos came down in bad weather in a residential area of the port city of Santos, in Sao Paulo state.
Local reports say four other passengers and the two pilots were also killed.
President Dilma Rousseff has declared three days of national mourning and cancelled campaign events.
Dilma Rousseff’s Vice President, Michel Temer, also expressed his regret over the death of Eduardo Campos, who had been running third in the polls for October’s election.
Eduardo Campos’ family had actively opposed Brazil’s military rule between 1964 and1985.
A married father of five children, Eduardo Campos, 49, served two terms as governor of the north-eastern Pernambuco state.
His vice-presidential candidate, Marina Silva, a popular politician and a former government minister, was reportedly meant to have been on the plane with him but decided not to travel.
Eduardo Campos was flying from Rio de Janeiro to Guaruja airport, outside Sao Paulo, when it hit bad weather, aviation authority spokesman Pedro Luis Farcic said.
According to air force officials, air traffic control lost contact with the plane after it was unable to land at Guaruja due to bad weather.
It plunged into several houses in a densely-populated neighborhood, sending plumes of smoke into the air.
Local residents said the sky was cloudy and it was raining at the time of the crash.
All seven people aboard the plane, including a campaign photographer and cameraman, a press adviser, as well as two pilots, died in the crash, City Hall press officer Patricia Fagueiro told the Associated Press news agency.
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