Most Helpful Girl
Yes, they should. The endorsement would be for the leader's desired candidates that would be his/her choosing, anyway. Plus, it's only an endorsement that can be considered or disregarded by the endorser's followers; it's not a direct invite for the subject of endorsement to hold office.
That is also an expression of support, something that cannot be taken away from them especially in a country that guarantees freedom of expression.1
Most Helpful Guy
Current political leaders cannot be prevented from endorsing candidates in elections, so it really doesn't matter whether it is a wise thing to do. They are going to do it anyway.1