Fossil fuels are key.
Greenberg ’16 [Scott; July 5; business & franchise speaker, author of multiple books; “Tax Proposals in the 2016 Democratic Platform, Explained,” Tax Foundation, https://taxfoundation.org/tax-proposals-2016-democratic-platform-explained-part-1/]
The federal tax code contains several provisions that treat oil, gas, and coal companies differently than other businesses. Most of these provisions allow fossil fuel companies to deduct more of their expenses up front, leading to lower taxes. The Congressional Research Service estimates that, in 2013, the federal government lost $4.8 billion due to tax preferences for fossil fuel companies (compared to $13.4 billion in tax preferences for renewable energy).
Both Clinton and Sanders would eliminate all tax preferences for fossil fuel companies. In addition, both candidates would also limit the ability of certain fossil fuel companies to claim foreign tax credits for their business activity overseas. The Democratic platform reflects the consensus of the two candidates on this issue.