There's a lot wrong with the way American democracy functions these days, but the biggest problem is money. Limits on spending, some in place for a century, have been thrown out by the Supreme Court. Much of the increased spending is what's called "dark money" because the sources are not transparent. There's good background in an article in The Atlantic. It cites a Brennan Institute study that finds a surge in semi-secret outside spending since the Citizens United case in 2011.
What's especially worrisome to me is how special interests can target state and local races with huge and unexpected -- and often highly misleading -- negative ads. The article gives some examples of out-of-state groups hiding behind "good government" names swamping incumbents.