All seven people aboard the plane, including a campaign photographer and cameraman, a press adviser and two pilots, died in the crash, Santos City Hall press officer Patricia Fagueiro told The Associated Press.
She said six adults and a baby on the ground suffered non-life-threatening injuries, but rescue officials said later that five people were slightly hurt.
In a solemn address, President Dilma Rousseff declared three days of official mourning in honor of Campos and said she would suspend her campaign during that time.
"Today Brazil is in mourning and reeling from a death that took the life of a promising young politician," she said, adding that Campos had been facing "an extremely promising future."
Campos, the scion of a political family from the northeastern state of Pernambuco, had been an ally of Rousseff but broke away ahead of the campaign for the Oct. 4 presidential election.
Polls suggested he was running in third place, far behind Rousseff and another political rival. But his Brazilian Socialist Party ticket was widely regarded as among the best-placed to challenge Rousseff and her Workers Party, thanks largely to his popular running mate, former Environment Minister Marina Silva, who joined Campos' ticket after her attempt to run for president herself failed.
It was not immediately clear whether Silva would assume Campos' spot as the party's presidential candidate. Under Brazilian law, in the event of a candidate's death, a party has 10 days to decide on a substitute.
A visibly shaken Silva spoke to reporters in Santos, focusing in her brief remarks on her relationship with Campos and giving her political future a wide berth.
"During these 10 months of partnership, I learned to respect him, admire him and feel confidence in his attitudes and his ideals in life," Silva said in a soft, wavering voice. She rose from the microphone before the press could pepper her with questions.
Pundits said Campos' death could complicate the presidential race for Rousseff.
Brazilian television broadcast a continuous loop of images of the wreckage, a smoldering pit between buildings of several stories, with emergency workers picking through the rubble. Brazil's top broadcaster, Globo, ran interviews with eyewitnesses who reported the plane was already ablaze before the crash around 10 a.m.
The plane took off from Rio de Janeiro, where Campos appeared in a television interview Tuesday, and was headed to the city of Guaruja, where he was to participate in a conference about Brazil's ports. Aeronautical authorities said the Cessna 560XL was attempting to land in bad weather.
Ana Lucia Domingues, who lives near the crash site, said she was making breakfast for her daughter when she heard the sound of a low-flying aircraft.
"It was a very strong buzz then a bang that even made our door shake. We thought it can't be. A helicopter, a jet?" she said, adding that she was a Campos supporter. "I can't believe our candidate was there. Nothing like that ever happens. It's too confusing."
The country's top politicians expressed shock and sorrow over the accident, with former President Luiz Inacio Lula da Silva calling Campos "a public figure of rare and extraordinary quality."
"The affection, respect and mutual admiration were always present in our dealings with one another," Silva said in a statement.
As the initial shock of the crash subsided, pundits began to speculate about how Campos' death would affect the presidential race.
David Fleischer, a political scientist at the University of Brasilia, said Campos' death was "bad news for Brazil and very bad news for Dilma."
Should Campos' running mate assume the candidacy, she would likely pull votes away from Rousseff, forcing the race into a second-round ballot between Rousseff and the other main candidate, Aecio Neves of the Brazilian Social Democracy Party. Silva's support of Neves in a runoff could threaten Rousseff's chances of re-election, Fleischer said.
Rousseff, the hand-picked successor of ex-President Silva, has seen her popularity flag in recent months amid popular dissatisfaction with slowing growth, high taxes and poor public services - although she has remained the strongest candidate. A survey by the Datafolha polling agency released Wednesday before the accident said 8 percent of those questioned said they intended to vote for Campos, compared with 36 percent backing Rousseff.
Campos, 49, was married and the father of five children, the youngest of whom was born in January with Down syndrome. The heir of a political dynasty that stretched back to his grandfather, he served two terms as governor of Pernambuco state.
His brother, Antonio Campos, told Globo and other broadcasters that Campos would be buried in the family tomb in Pernambuco, where his grandfather's body lies. The grandfather, Miguel Arraes, died on the same date, Aug. 13, nine years ago.
Associated Press writer Adriana Gomez Licon reported this story in Santos and Stan Lehman reported from Sao Paulo. AP writer Ana Santos in Rio de Janeiro contributed to this report.