Both parties are beholden to special interests, but they're usually different special interests.
A quick perusal of public filings for 2016 campaign contributions shows that democrats disproportionately received more money from unions (teachers, carpenters, nurses, restaurant workers, etc.), while republicans received more from casinos, private prisons, energy companies, and other corporations.
AT&T gave $7.7M to republicans for 2016, and $2.3M to democrats. Comcast was more balanced with $3.3M and $3.7M for republicans and democrats respectively.
There are certain sectors that give a lot of money to both parties. Hedge funds, insurance companies, and pharmaceutical companies are a good example, which might explain why we pay so much more for drugs than most countries, and why we've been so resistant to any form of single payer healthcare.
Political campaigns are long, drawn-out affairs in the US. The campaign for 2016 kicked off in the latter half of 2015! Running a campaign for that long costs a lot of money, and it all has to come from somewhere. Unless we're willing to get serious about campaign finance reform, or dramatically shortening campaign cycles, our politicians are going to continue to owe favors to special interests.
https://www.opensecrets.org/orgs/list.php