New Jersey Gov. Chris Christie (R) on Thursday announced he will appoint state Attorney General Jeff Chiesa to the seat left vacant by the death of Democratic senator Frank Lautenberg.

Chiesa will serve for only a few months until a new senator can be chosen in a special election on Oct. 16, a date chosen by Christie.

Christie said Chiesa will be sworn in Monday after the necessary paperwork is filed.

Chiesa’s name came largely as a surprise. In the hours before his selection, many New Jersey politicos thought Christie, a potential 2016 presidential candidate, would pick former New Jersey attorney general David Samson.

The new U.S. senator is a longtime ally of Christie, having served with him during Christie’s time as U.S. attorney and heading the newly elected governor’s transition team in 2009.

He was appointed as attorney general by Christie in January 2012.

Chiesa confirmed he will not seek the Senate seat in the special election. He said he is registered as a Republican and that he thinks he registered as such in 2000.

Asked about his position on immigration — one of they key issues he will likely vote on — Chiesa said he is a border-security-first Republican.

“I think the first thing we have to do is make sure the borders are secure,” he said. “From there, these issues are new to me.”

Chiesa was quick to acknowledge the limits to what he can accomplish in a short stint in the upper chamber.

“What do I hope to achieve? I’ve got a limited time period to contribute. I will try to contribute in every way that I can,” Chiesa said. “But I don’t kid myself into thinking I’m going to go down there with a handful of draft bills and start moving them as soon as I get there.”

A 1987 graduate of the University of Notre Dame, Chiesa earned a law degree from the Catholic University of America in 1990. He lives in Branchburg, N.J., with his wife, Jenny, and their son and daughter, according to a biography provided by Christie’s office.

Sean Sullivan contributed to this report.