JERUSALEM — The prime minister of Israel, Benjamin Netanyahu, announced Thursday that his conservative Likud Party would run on a joint ticket with the nationalist Yisrael Beiteinu Party in January elections. The surprise joining of forces immediately shook up Israel’s political map and was apparently intended to cement Mr. Netanyahu’s chances of leading the next government.

The move sharpened the contours of the left and right camps in Israeli politics after years during which the major-party leaders, including Mr. Netanyahu, had gravitated toward the political center. Political opponents from the center and left warned that the unification of Likud and Yisrael Beiteinu, led by Foreign Minister Avigdor Lieberman, reflected a creeping extremism that would not serve Israel.

By joining up, Mr. Netanyahu and Mr. Lieberman clearly intended to bolster their tickets and guarantee their leadership of a strong governing coalition in the coming years.

“This joining of forces will give us the strength to defend Israel and the strength to make economic and social changes within the state,” said Mr. Netanyahu, standing alongside Mr. Lieberman at a televised news conference timed to be broadcast live on the evening news programs.

Citing the great challenges facing Israel from within and without, including the threat of a nuclear Iran, Mr. Netanyahu said, “Together, we will seek a mandate from the public to lead Israel powerfully in the coming years.”

Mr. Lieberman, a Russian speaker who immigrated to Israel from Moldova in 1978, is a blunt-talking politician whose party has advocated some contentious and populist policies, like a demand for a loyalty oath in Israel because of concerns about the country’s Arab citizens.

“We have chosen the option of national responsibility,” Mr. Lieberman said at the news conference.

In Israel’s splintered political scene, splits and mergers are common, and the plethora of parties representing different sectors of the population and interest groups has produced unstable coalitions for decades.

In the current 120-seat Parliament, Likud and Yisrael Beiteinu together command 42 seats, and sit together in a coalition made up of six parties with close to 40 ministers and deputy ministers. If the new superparty were to garner a similar number of mandates in the coming elections, or more, it would be able to form a coalition that would be dependent on fewer parties.

There has been feverish speculation in Israel of late that the only possible challenge to Mr. Netanyahu would come from a return to politics by Ehud Olmert, the former prime minister, as the leader of a newly formed coalition of centrist and leftist forces. Bogged down by legal problems, Mr. Olmert has not yet announced his intentions.

Many commentators said the new rightist merger would neutralize any threat from such a coalition of center and leftist parties.

Others saw the merger as a risky gamble that could potentially benefit the opposition.

“In the last campaign for the 2009 elections, Netanyahu played the center ticket, and that was the policy of his government,” said Abraham Diskin, a political scientist at Hebrew University and the Interdisciplinary Center in Herzliya. “The risk now is of deterring centrist voters.”

But commentators noted that Mr. Lieberman, by becoming No. 2 in the newly merged list, was inching closer to his ultimate goal of one day leading the right wing in Israel.

“Lieberman is assuming the position of Netanyahu’s successor, a move he has been dreaming about for years,” said Raviv Druker, a political analyst, on Israel’s Channel 10 television.