Opposition parties across the political spectrum have joined forces, potentially denying Orban's right-wing Fidesz party a landslide win for the first time in almost a decade.
Since Orban came to power in 2010, he has overhauled election rules, making it more difficult for opposition parties to compete individually against Fidesz.
The main battleground is the capital Budapest, where Gergely Karacsony, 44, is neck-and-neck in polling for the mayor's post with the Fidesz-backed incumbent Istvan Tarlos, who like Orban has been in office since 2010.
Karacsony has compared the Budapest race to the Istanbul mayoral election in March, in which the candidate of Turkish leader Recep Tayyip Erdogan's party was defeated by the opposition challenger.
"Istanbul voted against an aggressive illiberal power in many ways similar to Orban's regime," Karacsony told AFP.
After sweeping to power, Orban pushed through swathes of new legislation, including measures that critics at home and abroad say have neutered key democratic institutions such as the media and the judiciary in the EU member state.
Critics say some of those reforms helped Fidesz to rout opposition parties at parliamentary, European and local elections despite around half of the electorate often voting against it.
Now, in many municipalities just one opposition challenger is lining up against Fidesz.
Outside the capital tight races are also forecast in several currently Fidesz-run cities like Miskolc, Pecs, and Szombathely, as well as in most of Budapest's 23 district municipalities.
Hopes of opposition gains increased last week after a sex scandal involving a Fidesz mayor erupted, threatening to erode the party's support nationwide.
An anonymous blogger posted explicit pictures and videos of the mayor of the western city of Gyor taking part in an orgy on a yacht in Croatia.
Fidesz leaders, who brand the party as a Christian-conservative one promoting family values, have sought to downplay the scandal. But analysts say its impact on voters is still unclear.
Around 1,000 people demonstrated against Fidesz in Gyor late Saturday, with opposition politicians from different parties urging the crowd to vote out the mayor.
Observers see the elections as a litmus test for the opposition's new strategy of cooperation, which could offer a way to challenge Orban in the next general election in 2022.
The opposition is expected to make gains Sunday from its 2014 results and possibly win the Budapest mayoralty. But it needs to do well in the countryside to stand a chance in 2022, says Andras Biro-Nagy of the Policy Solutions think-tank.
"Budapest has symbolic value of course, but it is equally important to win provincial cities and build roots and infrastructure there, that is where 2022 will be won or lost," he told AFP.