An organization founded by former Vice President Mike Pence is ramping up a pressure campaign to convince Republican senators to reject Robert F. Kennedy Jr.’s nomination to become President Donald Trump’s Secretary of Health and Human Services.

Pence — who served as Trump’s vice president in his first term — is opposed to RFK Jr. due to his previous support for abortion.

Whatever the merits of RFK Jr’s Make America Healthy Again initiative—indeed, whatever other qualities a nominee might possess—an HHS Secretary must have a firm commitment to protect unborn children, or else bend under the pressure and pushback surrounding these daily, critical decisions,” Advancing American Freedom President Tim Chapman and Board Chairman Marc Short, Pence’s former chief of staff, wrote in a letter to senators.

“While RFK Jr. has made certain overtures to pro-life leaders that he would be mindful of their concerns at HHS, there is little reason for confidence at this time,” they wrote.

This marks the second formal effort by Pence and his organization to block Kennedy’s confirmation. Shortly after Trump announced Kennedy’s nomination, Pence issued a statement urging senators to reject him due to his past support for abortion, The Hill reported.