Budapest Mayor Gergely Karacsony said at a news conference that he was bringing his candidacy for prime minister to an end, and asked his supporters to get behind his opponent, moderate conservative Peter Marki-Zay, in the interests of “reuniting Hungary.”

Karacsony’s withdrawal came after a week of back-and-forth negotiations with Marki-Zay, the independent mayor of a small Hungarian city, over which of the two men had the best chance of winning the second round of the primary set to begin Sunday — and then defeating Orban in a national election next spring.

Though the liberal Karacsony had several disagreements with Marki-Zay on policy, he said, “I came to the conclusion that if I do not (step aside), then Viktor Orban will remain, and the significance of the political debates between us will go up in smoke.”

“I believe that Peter Marki-Zay can unite the opposition,” he said.

The two-round primary race is part of a hard-won strategy by Hungary’s six main opposition parties to put aside ideological differences and mount a single challenger to Orban after more than a decade of bitter losses to his Fidesz party, which holds a two-thirds majority in parliament.

Total unity is the only method, they say, to overcome an electoral system authored by Orban’s party that gives it an unfair advantage.

Their coalition includes green, liberal and right-wing parties, as well as Marki-Zay’s “Everyone’s Hungary Movement,” an independent civic political initiative.

Despite coming in third place during the first round of the primary, receiving 7% fewer votes than Karacsony, Marki-Zay has insisted that only he can mobilize both liberal and conservative Hungarians — and disaffected Fidesz voters — who want an end to Orban’s rule.

Marki-Zay will now face center-left candidate Klara Dobrev of the Democratic Coalition party, who won the most votes in the first primary round.

Dobrev, a 49-year-old lawyer and economist and a vice president of the European Parliament, has campaigned on dismantling Orban’s political system and challenging Hungary’s 2012 constitution, which was unilaterally authored and passed by his party.

Both Karacsony and Marki-Zay have argued that Dobrev will be unable to defeat Orban in a national election, due in part to the fact that she is married to former Prime Minister Ferenc Gyurcsany, one of Hungary’s most divisive and unpopular politicians.

Hungary under Orban has “lost its way,” Marki-Zay said at Friday’s news conference, and become “a corrupt dictatorship from which people are fleeing.”

The coming election, he said, “is about whether we will be free and whether Hungary will be European, or whether it will sink into the swamp.”