She said in a meeting in Iowa that it was “a distraction from the important work we have to do to combat climate change”.
Unions and oil giants lobbied for the plans to go ahead because it would create thousands of jobs.
Clinton also said she would take on “price gouging” in the pharmaceutical industry by introducing a $250 (£163) monthly cap on prescription drug costs.
Citi biotechnology analyst Liav Abraham said that with candidates taking on steep drug pricing, “companies with less differentiated, more concentrated product portfolios are likely to come under political scrutiny”.
The NYSE oil index closed down 1.41 per cent, while the biotech index was down 1.45 per cent.