By Brad Haynes

SAO PAULO (Reuters) - Millionaire media mogul João Doria's stunning entry into Brazilian politics with a crushing first-round victory in Sao Paulo's mayoral race has fanned mounting speculation about a presidential run by his main political supporter.

Geraldo Alckmin, governor of Sao Paulo state, backed the political newcomer against resistance from nearly every other powerbroker in his Brazilian Social Democratic Party (PSDB), and in doing so undercut rivals for the presidential nomination in 2018.

Capturing 53 percent of valid votes in Brazil's biggest city on Sunday, Doria demolished a re-election bid by Mayor Fernando Haddad, who took less than 17 percent as his Workers Party suffered fallout from a vast corruption scandal and the impeachment of former President Dilma Rousseff.

That clinched the Sao Paulo race in a single round of voting for the first time under election rules that began in 1992. The first-round win surprised even Doria's allies, capping his climb from single-digit support after Alckmin offered his endorsement.

The former star of Brazilian TV show "The Apprentice" and publisher of Caviar Lifestyle magazine bankrolled much of his campaign with a personal fortune of 180 million reais ($56 million), drawing comparisons in a bruising primary race to U.S. Republican presidential candidate Donald Trump.

But Doria likened his run to another wealthy U.S. businessman's late entry into politics, promising to run city hall with the technocratic style of former New York Mayor Michael Bloomberg.

"I'm not a politician. I'm a businessman and administrator," Doria declared throughout the campaign, highlighting his work building a media and conference conglomerate from nothing.

His outsider pitch connected with voters growing sick of a political class stained with bribery allegations, but it also played down the careful political groundwork laid by Alckmin.

The four-term governor assembled a 13-party coalition to back Doria's bid, giving him more TV airtime than any other candidate in Sao Paulo. Television advertising, apportioned strictly under election law, is more valuable than ever after recent limits on corporate campaign donations.

Story continues

Voters may be tired of traditional candidates but party structures remain crucial to campaigns, according to senior analyst Carlos Bellini of political consultancy Arko Advice.

"I would say there is room for more names to appear like Doria's, entering traditional parties with the rhetoric of efficient administration," he said.

BULLDOZING CONSENSUS

Few political parties in Brazil are as traditional as the PSDB, and Sao Paulo is home to its most prominent elder statesmen.

From former president Fernando Henrique Cardoso to Foreign Minister Jose Serra, they lined up this year behind the mayoral campaign of city councilman Andrea Matarazzo, a former ambassador to Italy and communications minister under Cardoso.

Only Alckmin, who has built a political base over decades in the smaller cities of the state's interior, dared to challenge the consensus among the party's elite, and he was the only one among them celebrating Doria's victory onstage on Sunday night.

Senator Aecio Neves, the PSDB candidate who lost Brazil's 2014 presidential race by 3 percentage points, told CBN radio on Monday that it was too soon to discuss the next campaign.

Still, he recognized that it was an open race within his party. The PSDB has nominated Neves, Serra and Alckmin in the three most recent presidential campaigns.

"In 2018 our party will be even stronger, and our candidate will be whoever has the best chance to win," he said.

($1 = 3.23 reais)

(Reporting by Brad Haynes; Additional reporting by Maria Carolina Marcello in Brasilia and Eduardo Simoes in Sao Paulo; Editing by Frances Kerry)