NEW YORK (AP) — The longtime head of Regnery Publishing, one of the country's oldest conservative book publishers, is stepping down.

Regnery announced Thursday that Marji Ross will retire effective Jan. 1 as president and publisher. She joined the Washington, D.C.-based company in 1999 and has served in her current position since 2003. Ross will be succeeded by Regnery senior editor Tom Spence, who runs Regnery's Gateway imprint.

"This decision is a bittersweet one for me. The last 20 years have been the most enjoyable and rewarding of my professional life," Ross said in a statement. "I'm so grateful that Tom has agreed to take on this role. I know he will be a fantastic leader."

Regnery's history predates the modern conservative movement. It was founded in 1947 and has released such conservative landmarks as William F. Buckley Jr.'s "God & Man at Yale" and Russell Kirk's "The Conservative Mind," along with newer works by Ann Coulter, Newt Gingrich and Michelle Malkin, among others. The Regnery publication "Justice on Trial: The Kavanaugh Confirmation and the Future of the Supreme Court" made bestseller lists over the summer.

A division of Salem Media Group since 2014, Regnery has faced increased competition over the past 20 years as mainstream publishers began adding conservative imprints. One of Ross' predecessors at Regnery, Al Regnery, recently helped found the conservative Republic Book Publishers.