Lindsey Graham:
"He has a lot of sway over the Republican Party. If he objects to anything Biden [does], it would be hard to get Republicans on board. If he blessed some kind of deal, it would be easier to get something done. In many ways, he’ll be a shadow president."
>https://nymag.com/intelligencer/amp/2020/12/lindsey-graham-predicts-trump-will-become-shadow-president.html
UK definition of Shadow Cabinet:
The Shadow Cabinet is the team of senior spokespeople chosen by the Leader of the Opposition to mirror the Cabinet in Government. Each member of the shadow cabinet is appointed to lead on a specific policy area for their party and to question and challenge their counterpart in the Cabinet. In this way the Official Opposition seeks to present itself as an alternative government-in-waiting.
>https://www.parliament.uk/site-information/glossary/shadow-cabinet/