Opinion polls had indicated in the run-up to the poll on Tuesday that Kadima, led by Ehud Olmert, would win about 34 seats, enough to form a governing coalition in the 120-member parliament.
But turnout by 8pm local time (1800 GMT) on Tuesday was only 57% of registered voters, the lowest in Israeli history at that stage. Analysts said Kadima would be hit hardest if voters stayed away in large numbers; low turnouts at elections tend to favour more extreme parties whose supporters are more driven.
Ariel Sharon, the Israeli prime minister, founded Kadima last November before he suffered a stroke and went into a coma.
Media exit polls will be issued after balloting ends at 10pm and near-final results should be available early on Wednesday.
Israelis voted on the same day the Palestinian parliament approved a Hamas government. The group, formally sworn to Israel's destruction, called for a "just peace" on Monday but has shown no sign of softening its stance on the Jewish state.
About 20,000 police and volunteers were on patrol to guard against possible Palestinian bombings on election day.
For Olmert, a win would represent a vote of confidence in "consolidation", his term for unilateral steps to set Israel's frontier by 2010 through the removal of remote West Bank settlements and the strengthening of bigger enclaves.
Palestinians see the plan as an attempt to deny them a viable state and annex land Israel occupied in the 1967 Middle East war.
Surveys in the week before polling day indicated that the centre-left Labour party, led by Amir Peretz, a former trade union leader, would take second place with about 21 seats, making it a probable coalition partner.
The right-wing Likud party, headed by Benyamin Netanyahu, the former prime minister, was tipped to take 14 seats. His followers say that removing more settlements would reward Palestinian resistance.
Amir Peretz's Labour may join a
But Shaul Mofaz, the defence minister and Kadima politician, said in election-eve comments on Israel's NRG internet site: "It is a plan to determine our own fate if there is no peace partner on the Palestinian side."
A policy of unilateralism could spell the end of the so-called road map, which envisages a cessation of violence and the start of mutual steps leading to the creation of a Palestinian state alongside a secure Israel.
Neither side has met its commitments under the blueprint sponsored by the quartet of Middle East peace brokers - the US, the European Union, the UN and Russia.