Trump's ego is so big I can't grasp him in that role-but it appears that is what is happening. - Tom
A stalking horse is a person who tests a concept with someone or mounts a challenge against them on behalf of an anonymous third party.
If the idea proves viable and/or popular, the anonymous figure can then declare their interest and advance the concept with little risk of failure.
If the concept fails, the anonymous party will not be tainted by association and can either drop the idea completely or bide their time and wait until a better moment for launching an attack.
Or Trump, a long-time Hillary supporter and donor, is this year’s Philip Berg, esq. or Linda Starr perhaps?