I want to become POA to protect my father, but he will fight me over this. What can I do?

Power of Attorney is something that your dad would have to grant you, of his own free will.

You can gain guardianship over him by taking him to court and having him found incompetent. Not an easy process, nor a cheap one.

What kind of bad financial decisions is he making? Does he have a lawyer/financial adviser/ clergyman, i.e., someone he trusts, who can be your ally in getting him to consider better choices?