In 2010, the Supreme Court ruled in Citizens United v. FEC that corporations have the same free speech rights as people, and that spending money on elections is a form of speech. That means corporations can spend unlimited amounts to influence elections. That turns elections into auctions.
Corporations are not people. They do not vote, they do not serve in the military, they do not send their kids to public schools. What they do have is money, and a lot more of it than any individual voter. If political influence is proportional to dollars spent, then corporations will always have more political power than the people.
The result is campaigns that cost hundreds of millions of dollars, most of it raised from a tiny number of mega-donors and corporate interests who either want to curry favor or want a job in the future administration. That money does not make anyone's life better. It pays for ads, consultants, and attack mail. Every billion dollars spent on an election cycle is a billion dollars that went to convincing people instead of helping them.
Overturning Citizens United will require either a new Supreme Court ruling or a constitutional amendment. Neither is easy. That is exactly the kind of hard, structural fight that belongs on a platform, because the politicians who benefit from the current system will never fix it on their own.
Campaign finance reform is not a left issue or a right issue. Polls consistently show that large majorities of Americans across the political spectrum think there is too much corporate money in politics. The only people who like the current setup are the ones writing the checks.