They pay for ads which generate votes.
They could vote based on something else but they are usually so concerned about a democrat/republican getting in that they'd rather vote for a republican/democrat.
When I buy ads (donate) I buy ads I buy for the Green Party in areas where they are splitting the left wing vote.
The best way I know of to influence government is to have professionals prepare my taxes, and to ensure the government follows the law to the letter. Voting is designed to limit the influence of the individual on politics while appearing to give people choice.
If we wanted government representative of the views of the people we'd just draw 500 names of citizens out of a hat.