By Wiktor Szary and Pawel Sobczak
WARSAW (Reuters) - In a televised debate in Poland in 2007, center-right leader Donald Tusk accused election rival Jaroslaw Kaczynski of once pulling a gun on him in a lift.
"Killing you would be as easy as spitting," Tusk, who is now president of the European Council, quoted Kaczynski as saying.
The conservative politician, the twin brother of late President Lech Kaczynski and the prime minister at the time, denied the accusation but lost the debate, widely seen as key to Tusk's Civic Platform (PO) winning the election.
The clash encapsulates the bitter rivalry between the two men which has shaped Polish politics for over a decade and continues to do so as the country of 38 million prepares for a parliamentary election on Oct. 25.
Both have a low profile in the campaign, though Kaczynski is running for parliament, but are the powerbrokers behind the scenes in an election which opinion polls suggest Kaczynski's Law and Justice (PiS) party will win, with his nominee, Beata Szydlo, set to take over from PO's Ewa Kopacz as prime minister.
"Of course, today the candidates for prime minister are Ewa Kopacz and Beata Szydlo but the truth is, when we say Kopacz, we are thinking Tusk, and when we say Szydlo, we are thinking Kaczynski," said Rafal Chwedoruk, a political scientist at Warsaw University.
"We are still living in the shadow of these politicians, and I don't think this is likely to change after the election."
The rivalry between Tusk and Kaczynski also enshrines a battle for supremacy between the conservative Catholic values espoused by the eurosceptic, economically left-leaning PiS and the center-right, pro-European course favored by PO.
A PiS victory could end an unprecedented eight years of political and economic stability overseen by PO following the divisive and difficult transition from communist rule in Poland, which for over four decades was in the Moscow-led Soviet bloc.
It could also herald an increase in the state's role in the economy and deepen divisions in the European Union over migrants as PiS officials have suggested they would oppose the relocation in Poland of people fleeing Syria and Iraq if they win power.
Opinion polls suggest the public is ready for change, with one poll this month putting PiS on 34 percent support, 10 percentage points ahead of PO.
Supporters see the church-going Kaczynski, who loved singing patriotic songs about Poland's struggle for freedom in his childhood, as the embodiment of the conservative values which they say are threatened by PO.
Tusk, 58, is held up by his backers as a perfect foil for his 66-year-old rival because of what they see as his poise, pragmatism and ability to keep his cool.
Their conspicuous absence from the campaign trail is for different reasons and has had contrasting consequences.
Wary of his polarizing image, Kaczynski anointed Szydlo, 52, as the PiS candidate for prime minister as she is more restrained as well as a veteran party loyalist.
Tusk has been in Brussels since December. Following his appointment to the top EU job last year, he also opted for a woman he trusts when he named Kopacz, then the speaker of parliament, as his successor as prime minister.
"Ewa has never let Tusk down, she has always been loyal to him," a PO insider said. "When Tusk is in (Poland) they get in touch, often conferring with each other."
Political analysts say Tusk's departure for Brussels and his choice of a lower-profile successor has contributed to the PO's decline in the polls.
Kaczynski's official backseat role has, by contrast, helped tone down the conservatives' rhetoric which has in the past irked some voters, pollsters say.
In Szydlo, he also opted for a woman who has tasted success in elections before - she oversaw fellow PiS member Andrzej Duda's successful bid to become president this year.
The public animosity between Tusk and Kaczynski goes back at least a decade though Tusk said in the televised debate in 2007 that the alleged incident with the gun, for which he gave no explanation, had happened nearly two decades earlier.
Tusk's centrists and Kaczynski's conservatives ran in the 2005 election expecting to be coalition partners, promising a moral and political renewal of Poland, at the time still hurting from its transformation from communism.
But the two parties turned against each other during a brutal campaign and failed to form a cabinet together.
Kaczynski became prime minister in 2006, when his brother was president, but lost the job to Tusk in a snap election after only 16 months in power. With Jaroslaw sidelined, Tusk feuded with his twin and political partner, Lech.
In a struggle for power, the two tussled over who would represent Warsaw at EU summits in Brussels in 2008, at the height of the global financial crisis, and argued over who should fly on the government's plane.
The conflict reached boiling point after a plane crash in Russia in 2010 in which Lech Kaczynski died alongside his wife and more than 90 others, including the central bank governor, top army commanders and other high-ranking officials.
Jaroslaw Kaczynski accused Tusk of being indirectly responsible for the crash, caused, in his view, by the government's negligence. Some opposition politicians have accused Tusk of obstructing an official investigation.
"I'd rather never have been born than build my political career on the dead's graves," Tusk said in response.
For all their animosity and political differences, both have roots in the anti-communist Solidarity movement of the 1980s.
And when Tusk was appointed European Council president last year, Kaczynski approached him in parliament to congratulate him, a move widely seen as a breakthrough in relations.
Commenting on what was thought to have been their first public handshake in years, Kaczynski said: "I told Prime Minister Tusk not to believe that I hate him."
(Writing by Wiktor Szary and Justyna Pawlak, Additional reporting by Paul Taylor and Alastair Macdonald in Brussels, Editing by Timothy Heritage)