WASHINGTON (CNN) — Citing health reasons, Rep James Saxton, R-N.J., announced Friday he plans to retire at the end of his term.

Saxton is the 13th House Republican to announce retirement this year. Two other House GOP members, Rep. Heather Wilson, and Rep. Steve Pearce, both from New Mexico, announced plans recently to run for the seat of retiring GOP Sen. Pete Domenici, in their state.

"Although I had intended to run in 2008 and was planning a strong campaign, developments which occurred earlier this year regarding my health have prompted me to make this decision," Saxton said in a written statement. He has been treated for prostate cancer.

Saxton, 66, is moderate Republican representing south New Jersey. He was re-elected to his 12th term in 2006 and is a senior member of the House Armed Services Committee. One Democratic candidate, John Adler, has already filed papers to run for Saxton's seat. Bush won the district narrowly in 2004.

A Democratic campaign aide argued the political environment is growing increasingly tough for moderate incumbents. "It's clear that Republicans are starting the feel the heat for their extreme positions on issues such as SCHIP and their close ties to George Bush," the aide said.

The Democratic Congressional Campaign Committee has run radio ads in Saxton's district targeting his opposition to SCHIP, the children's health-care bill.

Citing Republican gains of state and county-level seats in last week's election in the New Jersey district, a spokesman for the National Republican Campaign Committee maintained that Republicans hold an advantage in the district.

– CNN Congressional Producer Deidre Walsh