According to Rav Yosef, who signed a letter stating his psak, anyone encouraging others to ascend Har Habayit or who goes up is causing the public to sin and it is prohibited to vote for such a person. There are people, he says, who call themselves nationalist, from a variety of parties, who raise their hand up against God and His Torah and cause people to sin with the punishment of kareit, under the banner of nationalism.

Besides for the halachic problem itself of ascending Har Habayit, doing so also causes problems for us with other countries. Rav Yosef rejects the claim that it is needed for strengthening Israeli sovereignty and that, even if true that it strengthens sovereignty, is not any reason to permit an issur kareit..

Rav Yosef then says explicitly that it is prohibited to give such political parties any strength in the elections and to assist them or vote for them. ANyone who does will bear the sin of kareit and carry the collar of kareit around his neck.

source: Arutz7