Democratic presidential candidate Bernie Sanders is blocking President Barack Obama's nominee to head the Food and Drug Administration, the third senator to do so.

The Vermont independent said in a statement Tuesday that Dr. Robert Califf's ties to the pharmaceutical industry are too strong. Califf was a cardiologist and medical researcher at Duke University for more than 30 years.

Democratic Sen. Edward Markey of Massachusetts said Monday he had placed a hold on the nomination because the agency has not been tough enough on the abuse of opioid painkillers.

Republican Sen. Lisa Murkowski of Alaska also placed a hold due to concerns about the FDA's approval of genetically engineered salmon.

Margaret Hamburg left the job early last year. Dr. Stephen Ostroff is serving as acting head of the agency.