In 1904 Mouser ran for and was elected as a Republican to the 59th Congress. He successfully ran for re-election in 1906, serving in the 60th Congress.
Later career
He was an unsuccessful candidate for reelection in 1908 to the 61st Congress.
After the election, he resumed practicing law in Marion until his retirement in 1935. He also served as delegate to the 1908 Republican National Convention. From 1916 to 1925, he served as a judge in the Court of Common Pleas of Marion County.
Harding patrimony controversy
Mouser cross-examined Nan Britton in Britton's lawsuit (Britton v. Klunk), in which she claimed that the late U.S. President Warren G. Harding was the father of her daughter Elizabeth Ann Blaesing. Britton was unable to provide any concrete evidence, and was shaken by the vicious personal attacks made by Mouser, which cost her the case. Carl Sferrazza Anthony, author of Florence Harding, a biography of Harding's wife, wrote that court transcripts in Toledo, Ohio, show that Mouser referred to Britton as a "degenerate and pervert", and "brought (Florence Kling Harding) in by using Warren's 'love of his good wife' against a 'distorted... deranged... demented... [and] diabolical' Nan who had no respect for the marriage tie...."[citation needed]
DNA testing in 2015 confirmed that Blaesing was indeed Harding's daughter.[3][4]
Death
Mouser died in Marion, Ohio, May 6, 1949 and is interred in Marion Cemetery.