Outgoing Union Save Romania President Nicusor Dan | Andrei Pungovschi/AFP via Getty Images Leader of Romanian anti-corruption party resigns Union Save Romania leader said he didn’t want to put off conservative supporters of the party.

The head of Union Save Romania, which campaigns on an anti-corruption platform, on Thursday said he was quitting the party over its stance on marriage.

Nicușor Dan, whose party is the third-biggest in the Romanian parliament, said he was against party plans to oppose calls for the law to be changed to define marriage as only between a man and a woman.

He said the party had support from conservatives and progressives, and taking such a stance would betray the former.

"The worst thing that could happen in Romania now is that the main debate would not be about who steals and who doesn’t, but about who defends traditions and who doesn’t," Dan said in a press conference in Bucharest on Thursday.

He said he had not yet decided whether to remain as a member of parliament.

Last year, a conservative NGO called Coalition for the Family submitted a petition to parliament signed by three million people calling for a change to the constitution that would restrict marriage to heterosexual couples. The constitution currently defines marriage as the union between equal partners, without specifying their gender. Gay marriages and civil partnerships are, however, not legal. The parliament has debated the petition and a referendum may be held.

The party "still has a very fragile image as an anti-system, anti-corruption party," and he didn't want to put off potential supporters, Dan wrote to senior party members, according to Romanian media outlet Hotnews. Campaigning against the redefinition of marriage would turn it into a party of civil liberties, he wrote.

Dan, a mathematician, ran an unsuccessful campaign to become mayor of Bucharest in May 2016. He then quickly turned a Bucharest-based party into a national one and won almost 9 percent of the votes in general elections in December.

Union Save Romania's core members are former civil society activists and private sector workers who came together to fight corruption. Its members have taken different positions on topics such as civil partnerships and the separation of state and church.

Dan described the party as "a construction with many people. In this construction there is a majority that wants to go in a direction that I don’t think is good, but it’s their right to do it," he said.

After promising not to create a new party, Dan said he could return to the fold if the decision was overturned.