18 U.S. Code § 597 - Expenditures to influence voting

Whoever makes or offers to make an expenditure to any person, either to vote or withhold his vote, or to vote for or against any candidate; and

Whoever solicits, accepts, or receives any such expenditure in consideration of his vote or the withholding of his vote—

Shall be fined under this title or imprisoned not more than one year, or both; and if the violation was willful, shall be fined under this title or imprisoned not more than two years, or both.
(June 25, 1948, ch. 645, 62 Stat. 721; Pub. L. 103–322, title XXXIII, § 330016(1)(H), Sept. 13, 1994, 108 Stat. 2147; Pub. L. 104–294, title VI, § 601(a)(12), Oct. 11, 1996, 110 Stat. 3498.)

https://www.law.cornell.edu/uscode/text/18/597

What if the $100.00 deduction would be if that person in the state would just REGISTER to vote? (I'm thinking if they do that, they would vote?

I remember hearing that the key to the success of the Tesla car company was their ability to make money using a government program to give tax breaks to people who buy alternative fuel vehicles, and I would bet that if there was a $100 tax break on the table for registering to vote someone would find a way to register in all 50 states, or to receive the benefit somehow by registering other people. I guess that's not a good reason to NOT do it, just an observation. I'd still favor your tax break coming AFTER you vote, seeing as how so many registered voters never vote. Also, if you got it for registering in 2026 would you still get it again in 2028 for continuing to be registered, or would it be a one time deal?

Federal elections. THAT would catch people voting more than once?

Just use the same loopholes he did.

Whenever I hear a republican respond to the idea that tax breaks are gifts to the rich they always say, "...it's not a gift to allow someone to keep what is theirs..." or something like that. Well, it can cut both ways; not taking something from someone isn't the same as giving someone something.