Republicans need every vote they can get our else even their cheating won't work. He literally can't afford to alienate the people who threatened his life.
So much so that he'll set aside the ethic and moral standards he and all those "God-fearing, law-abiding, patriotic people in this country" profess to uphold.
A reasonable person with even a shred of self respect would simply choose not to run in such a situation. But then, if he was reasonable and had any self respect, he wouldn’t have chosen to be Trump’s VP in the first place.