Several Waltham politicians, including former mayor William Stanley and his son, state Representative Thomas Stanley, have endorsed Joe Kearns Goodwin’s bid to become the city’s next state senator. The elder Stanley, who served as Waltham’s mayor for 14 years, ending in 2000, and his wife, Roberta,  announced their endorsement of Kearns Goodwin in the Democratic primary race Thursday. Their son Thomas and state Representative John Lawn, a Watertown Democrat, did the same. They all cited Kearns Goodwin’s experience serving in Iraq and Afghanistan while in the US Army, and his ability to fight for the benefit of Waltham residents. Kearns Goodwin, of Concord, will be facing fellow Democratic candidates Mike Barrett of Lexington, Alex Buck of Chelmsford, Mara Dolan of Concord, and Joe Mullin of Weston in the primary election for the Third Middlesex District seat. Republicans Sandi Martinez of Chelmsford and Greg Howes of Concord are vying for their party’s nomination in the Sept. 6 primary.