Poles voted on Sunday for a new head of state after the death of president Lech Kaczynski in an air disaster, but his twin trailed the ruling party candidate in an audacious bid to take his brother's place.
Opinion polls have put Bronislaw Komorowski, 58, of the market-friendly Civic Platform ahead of Jaroslaw Kaczynski, 61, leader of the eurosceptic conservative Law and Justice party.
Poland was plunged into national grief by the April 10 plane crash in western Russia which killed Lech Kaczynski and 95 other Poles, including his wife Maria, senior aides, lawmakers and military top brass.
The government jet crashed as it came in to land at Smolensk to take the delegation to a ceremony marking the 70th anniversary of the World War II massacre of thousands of Polish officers by Soviet forces.
The snap election has political importance now though as since losing a parliamentary election in 2007, the conservatives had relied on Kaczynski's presidential veto powers to hamper policy moves by the liberal government.
Victory for Komorowski speaker of parliament and a close ally of Prime Minister Donald Tusk would end a policy logjam and boost Civic Platform before parliamentary elections in late 2011.
On top of the jet tragedy, the campaign has also been overshadowed though by floods in recent weeks that killed 24 people and forced thousands from their homes.
Both candidates have appealed to the 38 million Poles for national unity.
The campaign slogan of Jaroslaw Kaczynski -- usually known for his hardball style -- is "The most important thing is Poland". Komorowski is pushing for an end to infighting under the banner "Consensus is constructive".
Voters casting their ballots in the central Mokotow district of the capital Warsaw said that despite the tragic circumstances the poll was like any other.
"I chose Bronislaw Komorowski because of his experience in parliament and because Mr. Kaczynski did not pass the test when he served as prime minister -- he already had his chance," Grazyna Rykowa, 50, told AFP.
Agnieszka Bienacka, 40, said she had also picked the liberal.
"I voted for Mr. Komorowski because his party is tolerant and democratic, completely opposite from the populism his rival supports," she said.
Komorowski could take 41-51 percent of the vote, to Kaczynski's 29-35 percent, surveys show. The other eight candidates lag far behind.
If no one scores over 50 percent, a run-off between the top two will be held on July 4, with Komorowski the likely victor, polls indicate.
Under the constitution, parliamentary speaker Komorowski became acting president after the crash.
Even before the crash he was preparing to fight Lech Kaczynski's bid for a second five-year term in an autumn election, and was tipped to win.
Despite shared roots in Solidarity, the opposition movement that drove Poland's communist regime from power peacefully in 1989, Civic Platform and Law and Justice are bitter rivals.
"They're similar as candidates really, both of them with roots in the anti-communist opposition, but I voted for Jaroslaw Kaczynski -- he's more inclined to social welfare," said 25-year-old Michal Luczak.
The two parties have been at odds since 2005, when Lech Kaczynski beat Tusk in a tough presidential race.
In a unique duo, Jaroslaw Kaczynski was his brother's premier in 2006-2007, a period marked by regular clashes between the twins and fellow European Union leaders, plus bitter battles at home.
Civic Platform trounced Law and Justice in the 2007 general election.
Forced to work with Tusk, Lech Kaczynski did his best to block laws. He was wary of the government's drive to reform welfare and privatise state firms.
Tusk, who underscores that Poland bucked the global economic slump on his watch, has mended fences in the EU, which the country joined in 2004.
Polls close at 8:00 pm (1800 GMT).