This one a Republican (via Washington Post):

Michigan Rep. Vern Ehlers announced his retirement this morning, the 17th Republican to decide against seeking re-election to the next Congress.
“I don’t want to stay in office so long that people will say you should have left five years ago,” Ehlers said.
Extenuating circumstances likely played a role in Ehlers’ decision. His wife, Johanna, suffered a heart attack last week and, on Tuesday, conservative state Rep. Justin Amash announced plans to primary the 76-year-old Ehlers.
Ehlers’ 3rd district, which is centered in Grand Rapids, was extremely competitive in 2008 as Sen. John McCain (Ariz.) won it by just over 2,000 votes. The seat went strongly for George W. Bush in 2004 (59 percent) and 2000 (60 percent), however.