The interim senator appointed by Charlie Crist is aggressively backing the Florida governor’s rival in the Senate race.

Sen. George LeMieux (R-Fla.), who was once Crist’s chief of staff, formally endorsed Republican Marco Rubio on Monday.

LeMieux told the conservative blog The Shark Tank that he was “disappointed” in Crist’s decision to leave the GOP primary. “I’m supporting Marco Rubio, I’m a Republican,” he said. “We need folks who are going to be strong conservative voices and Marco’s going to do that.”

When Crist announced his decision to run as an independent in April, LeMieux said he would “support our Republican nominee.” But he had remained relatively quiet about his support for Rubio until now.

Rubio said LeMieux has been “very helpful.”

“Sen. LeMieux’s been very helpful to us, he’s contributed to us from his PAC and has offered his assistance and we’re grateful for that,” he said on a conference call with reporters.

Rubio also announced on Monday that he raised more than $4.5 million in the second quarter.