WASHINGTON (AP) — Senate Democrats’ campaign arm on Tuesday began a $3.6 million ad blitz against Republican Rep. Tom Cotton, starting with his vote against funding pediatric research at an Arkansas hospital.

Cotton, who is to face Democratic Sen. Mark Pryor in the fall, was the only member of the Arkansas delegation to oppose the National Institutes of Health establishing a National Pediatric Research Network that would include the Arkansas Children’s Hospital. Cotton and 26 other Republicans opposed the measure, which passed the House but remains stalled in the Democratic-controlled Senate.