Word for individual who tips the balance

Suppose there are two vice presidents vying for influence/supremacy within an organization.

One vice president is admittedly weaker than the other, so he forms an alliance with a powerful underling who can tip the balance and make him (his team) the stronger.

Is there a word for this third person, the underling on Team B, whose involvement would tip the balance?

I'm thinking of a politically-charged word like "power-broker", but in a more subservient role. (Power brokers pull the marionette strings, whereas this fellow is talent.)

Note: I would create the tag "mot-juste" but I presently lack the required rep.

Kingmaker, which wikipedia currently defines as "a person or group that has great influence in a royal or political succession, without being a viable candidate."

Using a political term it would be "swing voter". The swing voter doesn't have inherent power other than to tip the balance between the other, more significant, forces.

If you're talking specifically about a person added because they will tip the scales, you might try ringer:

a person who is highly proficient at a particular skill or sport and is brought in to supplement a team or group of people

This is normally used to say that the person came from "outside", so it may not be a perfect fit.