After nearly 90 per cent of votes were counted, Calin Georgescu, 62, was at 22 per cent, while Ciolacu had 21.7 per cent.
However, ballots from the sizeable Romanian diaspora, which are not included in the main tally on Sunday, show centre-right politician, Elena Lasconi, 52, first with 33.4 per cent and Georgescu second.
Romanian Prime Minister Marcel Ciolacu is fighting for his political survival. (AP PHOTO)
Romania's president has a semi-executive role with control over defence spending - likely to be a difficult issue as Bucharest comes under pressure to uphold NATO spending goals during Donald Trump's second term as US president while trying to reduce a heavy deficit.
Lasconi told supporters after exit polls were released giving her a small lead over Georgescu, with Ciolacu in first place, that she was optimistic about making the runoff.
"But as you can see, the results are very tight, let us wait until tomorrow's results to rejoice."
Campaigning focused largely on the soaring cost of living, with Romania having the EU's biggest share of people at risk of poverty.
Georgescu is a former prominent member of the hard-right Alliance for Uniting Romanians party.
Calin Georgescu is from the hard-right and isn't a fan of NATO. (AP PHOTO)
In 2021 he has called NATO's ballistic missile defence shield in the Romanian town of Deveselu a "shame of diplomacy" and said the North Atlantic alliance will not protect any of its members should they be attacked by Russia.
Lasconi, a former journalist, joined the Save Romania Union in 2018 and became party head this year. She believes in raising defence spending and helping Ukraine, and surveys suggest she would beat Ciolacu in a runoff.
Romania shares a 650km border with Ukraine and since Russia attacked Kyiv in 2022, it has enabled the export of millions of tonnes of grain through its Black Sea port of Constanta and provided military aid, including the donation of a Patriot air defence battery.
"It will be a tight run-off, with the Social Democrat leader more vulnerable to negative campaigning due to him being an incumbent PM," political commentator Radu Magdin said.Â
