JERUSALEM — Prime Minister 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 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 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.