BRASILIA (Reuters) - Brazil’s centrist parties, battered by widespread voter disaffection with the political establishment, must join forces to win the October elections in a dangerously polarized field, leaders from seven parties urged on Tuesday.

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A manifesto led by former president Fernando Henrique Cardoso called on centrists to unite to avoid the election becoming a fight between “radical, authoritarian and outdated” populists from the right and left.

Far-right congressman Jair Bolsonaro, with 25 percent of voter support, and leftist former governor Ciro Gomes, on 12 percent, led a poll released on Tuesday by news website Poder360, while more moderate candidates stagnated with single-digit support.

But the manifesto left open the question of who would lead a united centrist ticket, charged with defending the continuity of pro-market changes and fiscal austerity proposed by unpopular President Michel Temer.

“The question is, unite behind whom?” asked a strategist for Temer’s Brazilian Democratic Movement party (MDB), who asked not to be named.

The MDB backs former finance minister Henrique Meirelles, although he musters just 1 percent support. The party is counting on the millionaire to finance his own campaign, freeing up party funds for Congressional and other races, the party insider said.

The other main centrist force, Cardoso’s Brazilian Social Democratic Party (PSDB), backs former Sao Paulo governor Geraldo Alckmin, who remains stuck around 7 percent support, Tuesday’s poll showed.

As the leading centrist, Alckmin is best placed to top a united ticket, but he has so far resisted an alliance with the MDB. Allies say such a tie-up could risk his campaign being tarnished by association with Temer, whose rejection rate has climbed to 88 percent of voters.

Alckmin’s candidacy also divides his own party. Cardoso, who was president from 1995-2002, proposed that his party nominate TV presenter Luciano Huck instead.

Candidates must be registered by mid-August, with the parties set to decide their nomination by vote at a convention.

“The manifesto led by Cardoso is his way of saying he has no confidence in Alckmin’s candidacy and maybe it should be someone else,” said political scientist Leonardo Barreto of Brasilia-based consultancy Factual.

Both Bolsonaro and Gomes threaten the business-friendly policies that Temer has relied on to lift Brazil from its worst recession in decades, based on their track record of favoring state intervention in the economy.

“On the right, we have an unprecedented movement clearly inspired by anti-republican and anti-democratic ideas,” the centrist leaders wrote. “On the left, a vision of backward-looking utopias, authoritarian socialism and an interventionist and omnipresent state.”