I am creating an apparel brand and want to use the words "Donald Trump" in the company name. Is this illegal? I will be selling political apparel.
If you are Donald Trump posting under an assumed name then it is illegal, because he is currently president of the USA and not supposed to run any businesses.
If you are not Donald Trump, then be aware that Donald Trump owns a trademark for "Donald Trump" according to this site https://onlinetrademarkattorneys.com/donald-trump-trademarks/ so you will be infringing on his trademark.
A trademark is for a specific kind of thing. That's why, in his commercials, Bill Cosby keeps saying "Jell-O Brand Gelatin."
On a trademark application, you must list the Goods or Services for which the proposed trademark will be used.
"You may clarify or limit the goods/services but you may not expand or broaden the goods/services. For example, if you filed for “shirts,” you may limit the goods to specific types of shirts such as “t-shirts and sweatshirts.” However, you may not change the goods to “pants.” Likewise, if you file for “jewelry,” you may change the goods to specific types of jewelry such as “jewelry, namely, earrings.” However, you may not change the goods to a service such as “jewelry stores.”" https://www.uspto.gov/sites/default/files/documents/BasicFacts.pdf
It appears to me that as long as you don't use the name "Donald Trump" for the same goods/services that his copyright is for, there is no infringement. But if I were you, I would go ahead and apply for a trademark of your own. If the USPTO approves your trademark application for "Donald Trump Apparel" (or whatever) then you'll be way ahead of the game.