There's an opinion column in the Washington Post by David Byler titled "Politicians probably don't need your money." The gist is, once a candidate has enough money, more won't make a difference, and it's true. What does make a difference is contributing to grassroots organizations. If you're a U.S. citizen, may I recommend Blue Tent? Blue Tent tells you the most effective way to use your money, routing your contribution to the people who will put it to good use. https://bluetent.us/
Obviously, a lot of voters don't have money to donate to anybody or anything. But as Blue Tent points out, there's only so much that can be done with money. Helping out with local grassroots organizations can be far more effective than donated money. Blue Tent has information about these groups and honestly assesses their effectiveness. Here's a piece they published on what you can do after this past election. https://bluetent.us/blog/after-the-midterms-here-what-democratuc-donors-need-to-get-b/