Israeli Prime Minister Binyamin Netanyahu's ruling Likud party has approved an electoral pact with the ultranationalist Yisrael Beitenu.
Opinion polls predict that Monday's move will help the prime minister win elections in January.
Netanyahu had angered many Likud party faithful with his surprise announcement last week of the merger with Foreign Minister Avigdor Lieberman's party.
Some of Likud's 3,700-member ruling body tried to stall the vote by petitioning for a secret ballot, but the alliance won a quick show of hands after Netanyahu pledged in a speech that the move would "not change Likud" or supplant its leadership.
"I've got news for you, I intend to lead Israel for many years to come," he said to loud applause in a packed Tel Aviv auditorium.
Some in Likud had objected to the merger with Lieberman's faction, citing Yisrael Beitenu's widely-criticised legislative moves questioning the loyalty of Palestinian-Israelis and calls to investigate foreign funding of organisations, a move seen as targeting liberals.
Others worried that the deal, which divides up parliament seats between the parties, could rob them of re-election.
But many in Likud seemed swayed by opinion polls suggesting the new alliance would easily triumph in the January 22 national vote.
Summing up his own decision to back the union, Silvan Shalom, the vice prime minister, said on Israel Radio ahead of the vote: "It's true they didn't consult with us as they should, but now that the train has left the station, the risk of cancelling the deal is greater. The prime minister made a decision and it should be accepted."
Three surveys published on Sunday and Monday showed Netanyahu's new bloc winning 35 to 43 seats in the 120-member parliament, well ahead of 20 to 23 for left-of-centre Labour, and a dozen to 15 seats for other centrist parties.
The only challenger seen as possibly threatening Netanyahu's lead was ex-prime minister Ehud Olmert, former leader of the centrist Kadima party, who has not yet said whether he will run since his acquittal on most of the corruption charges that forced his 2008 resignation.
A poll published on Monday in the Maariv newspaper showed Olmert winning just 10 parliament seats to 42 for Netanyahu's Likud-Beitenu bloc.
The gap was narrower when Olmert's support was combined with that of other centrists such as former TV news anchor Yair Lapid, running as head of a separate party.
Some pundits see a possible comeback by former foreign minister Tzipi Livni as a potential wild card in the race. Livni quit parliament in March after losing a party leadership race in Kadima to Shaul Mofaz, a former general.
Livni and other centrists have been holding talks with Labour party leader Shelly Yachimovitch on the possibility of
forging a broader ticket against Netanyahu.
But most polls predict right-wing and religious parties more closely allied with Netanyahu will win a solid majority in the election.