Prime Minister Benjamin Netanyahu told supporters at a rally on Sunday that they were in a “fateful struggle”.
Mr Netanyahu faces a challenge from a centre-left alliance that has promised to repair ties with the Palestinians and the international community.
Neither side is expected to get more than a quarter of the votes.
Correspondents say the election is likely to be followed by a lengthy period of negotiations over the formation of the next coalition government.
Opinion polls published before the weekend suggest that the centre-left Zionist Union is likely to win the most seats.
However, the BBC’s Kevin Connolly in Jerusalem says that it might still be possible for Mr Netanyahu to form a coalition government even if his Likud party fails to top the poll.
Mr Netanyahu has consistently accused his centre-left challengers of being willing to relinquish Israel’s claim to Jerusalem as its indivisible capital in peace talks with the Palestinians.
“The important thing is to keep a unified Jerusalem,” he told Channel 2 news on Monday.
Palestinians seek East Jerusalem – occupied by Israel since the 1967 Middle East war – as the capital of a future Palestinian state.
In an interview with the Walla news website on Monday, Mr Netanyahu accused the opposition of being prepared “to bow their heads to any dictate, including to a nuclear deal [between world powers] with Iran”.
But Zionist Union party co-leader Yitzhak Herzog, whose alliance has been leading Likud in pre-election polls, has accused Mr Netanyahu of “panicking”.
Visiting the Western Wall, one of the holiest sites in Judaism, on Sunday, he pledged to “safeguard Jerusalem and its residents in actions, not just words, more than any other leader”.
Indications are that whichever party performs best, they could need to the support of new centrist party Kulanu, in order to build a coalition, though its leader, Moshe Kahlon, has not yet said who he will back.