if a political party or a belief gains ground, it becomes more popular or accepted.

The Republicans are gaining ground in the southern states. (often in continuous tenses)

• be on the wagon
• be on the warpath
• be on the wing
• be on the wrong end of
• be on the wrong end of something
• be on the wrong side of
• be on the wrong track
• be on thin ice
• be on to a good thing
• be onto a winner
• be on top of the world
• be on uppers
• be on your best behaviour
• be on your guard
• be on your last legs
• be on your mettle
• be on your tail
• be on your uppers
• be open to
• be open to abuse
• be open to [abuse/criticism etc.]
• be open to criticism
• be out after scalp
• be out after someone's scalp
• be out for blood