tighten (one's) grip (on someone or something)

To increase the intensity with which one exerts or assumes control over someone or something. The increased military presence and crackdown on dissidents makes it clear that the dictator is beginning to tighten his grip on the country. Kids are like sand—the more you tighten your grip, the more they wriggle out of your control. The CEO is mounting a massive legal campaign in an effort to tighten her grip on the company.