His likely main rival among the eight contenders for the largely ceremonial post is Dragan Primorac, backed by the conservative HDZ party that currently governs the country.
The election comes as the European Union and Nato member country of 3.8 million people struggles with biting inflation, widespread corruption and a labour shortage.
Milanovic is backed by 37% of the electorate while Primorac has the support of 20%, according to an opinion poll on Friday.
However, as none of the candidates are expected to garner more than 50% of the vote to win outright, the new head of state is likely to be chosen in a run-off in two weeks.
During the campaign the two main rivals often traded insults, with Milanovic ridiculing Primorac as boring and as “fake as a €13 note”.
Balance of power
Croatia’s president commands the country’s armed forces and has a say in foreign policy but, despite limited powers, many believe the office is key for the political balance of power.
Croatia has mainly been governed by the HDZ since independence in 1991.
“All the eggs should not be in one basket,” Nenad Horvat, a salesman in his 40s, told AFP.
He sees the current president as the “last barrier that all levers of power fall into the hands of HDZ”.
Milanovic, a former leftist prime minister, has been one of Croatia’s leading and most colourful political figures for nearly two decades.
Sharp-minded and eloquent, Milanovic, 58, won the presidency for the opposition Social Democrats (SDP) in 2020 with pledges to advocate tolerance and liberalism.
However, he used the office to attack political opponents and EU officials, often with offensive and populist rhetoric.
Milanovic, who condemned Russia’s aggression against Ukraine, has nonetheless criticised the West’s military aid to Kyiv.
That prompted Prime Minister Andrej Plenkovic to label him a pro-Russian who is “destroying Croatia’s credibility in Nato and the EU”.
Milanovic countered that he wanted to protect Croatia from being “dragged into war”.
“As long as I’m president no Croatian soldier will wage somebody else’s wars,” he said this month.
He regularly pans Plenkovic and his HDZ party over systemic corruption, calling the premier a “serious threat to Croatia’s democracy”.
“I’m a guarantee of the control of the octopus of corruption… headed by Andrej Plenkovic,” he said during the campaign.
President-PM feud
For many, the election is a continuation of the longstanding feud between two powerful politicians.
“This is still about the conflict between the prime minister and president. All the rest are just incidental topics,” political analyst Zarko Puhovski told AFP.
Primorac, 59, campaigned as a “unifier” promoting family values and patriotism.
The election will show “whether Croatia is turning towards East or West… towards divisions or unity”, he said.
A physician and scientist who returns to politics after 15 years, Primorac repeatedly accused Milanovic of “disgracing Croatia”, a claim that resonated with many voters.
Barbara Sente Ocvirk, 36, told AFP she was not “satisfied with the way our current president is representing us in Croatia and abroad” and believes his main rival would do better.
Yet critics say that Primorac lacks political charisma and credibility, and has served only as the HDZ’s attack dog to provoke Milanovic.
Voting stations open at 7am and close 12 hours later, when exit polls are expected.
Official results are due late on Sunday.