The conservative twin of the late Polish president was headed for a run-off yesterday with the moderate, pro-European leader who assumed the job after the plane crash that killed much of Poland's leadership.
The outcome is expected to shape the country's direction on a wide range of issues, including the adoption of the euro, welfare reform and its mission in Afghanistan.
Jaroslaw Kaczynski and interim President Bronislaw Komorowski will face each other on 4 July after neither was able muster the 50 per cent needed for outright victory.
Mr Kaczynski's policies are essentially identical to those of his deceased brother and he would be expected to pursue the same platform. Lech Kaczynski was killed along with his wife and 95 other people in the crash in Smolensk, Russia, on 10 April. Many called it the worst tragedy to strike Poland since the Second World War.
Mr Komorowski is a leading member of the pro-European Union, moderate Civic Platform party, which governs the country. He has pledged to work to adopt the euro in about five years, end the unpopular military mission in Afghanistan and promote pro-market reforms.
Observers say the close results should spur Mr Komorowski to invigorate his lacklustre campaign and seek new supporters. Former president Lech Walesa – a long-time foe of the Kaczynski twins – pledged his support.