The GIL party - Hebrew for age, but also the Hebrew acronym for Israeli Pensioners for Parliament - captured five to eight of the legislature's 120 seats, according to exit polls.
Its strong showing could hand Ehud Olmert, the acting prime minister, the politician likely to form the next government, the solid majority he needs to carry out his planned West Bank pullbacks.
GIL chairman Rafi Eitan, 79, was the Mossad agent who handled Jonathan Pollard, the US navy analyst convicted of spying for Israel.
At party headquarters on Tuesday, activists hoisted posters demanding that the US free Pollard, who has served 21 years of a life sentence.
The party platform does not mention Pollard.
But in a recent interview with the Yediot Ahronot newspaper's website, Eitan said his decision to recruit an agent to spy on Israel's staunchest ally was his decision alone.
"I took complete, absolute responsibility," he said.
"I decided to take the risk, which was clear to me, although I didn't predict the affair would develop to such an extent."
Eitan said on Tuesday that he would ally with any coalition that would address his constituency's issues.
"We will sit in any government or coalition that allows us to carry out our agenda and attain our goals of helping pensioners," he said.
"We will sit in any government or coalition that allows us to carry out our agenda and attain our goals of helping pensioners"
Kadima officials said Olmert would not have any problem forming a coalition because the pensioners would join.
But GIL's No 2 candidate, Yaacov Izri, said: "No promise was made to any party, to any person in any party. We are not in any party's pocket."
Politicians from other parties interpreted GIL's upset victory as a protest vote.
The party said it speaks for 750,000 Israelis of retirement age in this country of 7 million, but it apparently received a boost from voters not yet eligible to retire.
Natalie Ramati, 26, said she voted for the pensioners' party because it aimed to help people on the fringes of society.
"I try to vote for the least of the evils," said Ramati, who cast her ballot in Tel Aviv with her mother, who also went with the pensioners.
Shelly Yachimovitch, campaign head for Labour's Tel Aviv office, said on Tuesday she had noticed a pattern at Tel Aviv polling stations of people choosing to vote this year for GIL.
"I found myself convincing people to switch their vote from Pensioners to Labour, rather than from Kadima to Labour," she was quoted as saying on the website of the Haaretz daily.