Why do candidates need SO much money to run a successful campaign (meaning why do they spend all of this money on)?

They spend all of their money on Rallies, merch (signs, t-shirts etc.) Travel costs for going out of state to other Rallies and speeches, Advertising, staff, to help run for office.  

Explanation:

Political campaigns involve considerable expenditures, including travel costs of candidates and staff, political consulting, and advertising. ... The need to raise money to maintain expensive political campaigns diminishes ties to a representative democracy because of the influence large contributors have over politicians.

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