* Fidesz lost by-election in bastion near Serbian border
* Polls ahead of April 8 election put Fidesz ahead with 32 pct
* Leftist parties vie with Jobbik to lead opposition (Updates with comment from Orban)
By Gergely Szakacs and Krisztina Than
BUDAPEST, Feb 26 (Reuters) - Hungarian opposition parties said on Monday they will double down on their efforts to beat Prime Minister Viktor Orban at elections in April because of a shock defeat for his right-wing Fidesz party in a by-election on Sunday.
The main leftist opposition party, the Socialists, said they would seek to work more closely with other parties but did not want a structured alliance with Jobbik, a former far-right party that is projecting a more moderate image.
Instead, Jobbik should withdraw its candidates from constituencies where independent candidates stood a stronger chance, the Socialists’ candidate for prime minister, Gergely Karacsony, told a news conference.
“Our task is to widen the alliance,” Karacsony said.
Jobbik’s leader Gabor Vona said Sunday was a turning point. “This sends the message that Fidesz can be defeated in every part of the country,” he said in a video on Jobbik’s website.
The green liberal LMP party last week started talks with the Socialists and Jobbik on putting forward single candidates in constituencies where each party was strongest, said party co-chairman Akos Hadhazy.
“LMP is ready for compromise as long as our principles are upheld. The result (on Sunday) ... shows what we have held for a long time: only a credible candidate is worth our support,” he said.
Orban, 54, is Hungary’s longest-serving premier since the collapse of Communism. He presents himself as a saviour for Europe’s Christian nations and his party has an anti-immigration platform at odds with many of its European Union peers.
Fidesz is on 32 percent ahead of the April 8 vote, with Jobbik on 11 and the Socialists on nine, according to a poll in February by the Zavech Research institute, which showed 34 percent of voters are undecided.
But the race has been jolted by the by-election, in which an opposition-backed political novice easily beat the Fidesz candidate for mayor in Hodmezovasarhely, a rural Fidesz bastion.
The city by the Serbian border is near a steel fence built to keep out migrants, a hallmark of Orban’s policy.
“Our opposition has been dead and now has awakened,” Fidesz founding member Zsolt Bayer wrote in the pro-government Magyar Idok. He said it was Fidesz’s first serious defeat in a decade.
“We have never won an election by accident, for luck or the weakness of others,” Orban said in a video on the website of the weekly HVG. “If we want to win, our community has to make twice or three times the effort than our opponents.” Orban has clashed with the European Commission over migration policy and over reforms to the judiciary and media, which his critics say put Hungary on an authoritarian path.
Peter Kreko, director of think tank Political Capital, said Sunday sent a strong message to both the opposition and Fidesz.
“This came in time for the opposition to cooperate if they want to ... and also came in time for Fidesz to tweak its campaign,” he said. “It showed that there is not only a mathematical, but also a political chance to beat Fidesz.”
A broad opposition alliance could redraw the pre-election map in Hungary’s 106 constituencies, though it will be hard for the opposition to take advantage because it is fragmented, with leftist parties and Jobbik vying for supremacy.
“The opposition ... is no longer playing an old game where it stands no chance of winning, but a new game starts where its situation is not hopeless, provided it pursues smart tactics,” political analyst Gabor Torok wrote on his blog. (Writing by Krisztina Than; Editing by Matthew Mpoke Bigg)