Campaign expenditures enable candidates -- and groups that support them -- to reach out to voters directly, through expensive television advertisements as well as campaign literature. Candidates also use these donations to hire staffers and create campaign organizations that help to both persuade voters to support the candidate and to make sure that voters actually get out and vote on Election Day.
Many political scientists argue that people and the media often overestimate the impact of money on campaign success. But that doesn’t mean money isn’t very important. As the eminent political scientist John Sides has written, “The major debate is not over whether money matters, it’s over the relative impact of incumbent and challenger spending.”
Campaign spending does not, for instance, persuade Democrats to convert to Republicans, or vice versa. But it does enable candidates to educate voters on positive aspects of their candidacy and personality, as well as negative aspects of their opponents. In congressional races especially, this helps challengers offset the incumbency advantage, or the benefit that current officeholders have in seeking reelection due to voters’ familiarity with them through casework and local media appearances.
Writing in "The New Republic," political scientist Jonathan Bernstein says: “More generally, campaigns and campaign spending are only one of the things that affect how we feel about the candidates, which (remember) is only one factor in determining how we vote.”