Fidesz has been easily winning local, national and European Parliament elections since 2010, but a more unified opposition and the release of a video showing one of the party's best-known mayors, former Olympic champion gymnast Zsolt Borkai, participating in an orgy on a yacht shook up the last days of the campaign.
The sex scandal has visibly flustered Orban's party, whose officials initially called it a private matter. The conservative Fidesz, which since 2015 has made its reputation on anti-migration policies, also casts itself as a defender of Christian and family values.
In Hungary's most closely-watched vote, Budapest Mayor Istvan Tarlos, who is not a member of Fidesz but is backed by Orban's party, was running against Gergely Karacsony, a district leader in Budapest who is supported by five left-wing, Green and liberal parties.
More than 8 million people were eligible to vote for over 3,000 mayors and 17,200 local council members elected for five-year terms. Despite the scandal-ridden campaign — a secret sex video was also released about an opposition mayor and plenty of other shenanigans took place — the Hungarian government's dominant influence over large sectors of the media and its blatant warnings about dwindling financial support for cities that don't elect Fidesz mayors meant that opposition candidates were fighting mostly uphill battles.
On Sunday, Tarlos urged Borkai to step down, concerned about the fallout from the scandal. Borkai's actions were "insupportable, brazen, indefensible, vulgar and unworthy," Tarlos said on state radio. "The truth is that Borkai should resign, because it questions the achievements of others."
Borkai was to hold a news conference on Friday but abruptly cancelled it without explanation. Around the same time, Magyar Nemzet, a newspaper under close government oversight, published an online item calling for Borkai's resignation only for it to be removed within hours. The paper said the article did not reflect its views and was published by mistake.
Orban said he would wait until Monday to declare his position on Borkai, who has also been criticized for his business connections and alleged corruption. Polls close at 7 p.m. (1700 GMT, 1 p.m. EDT) and partial results are expected late Sunday.