He won election to the United States Senate in 1934, unseating Republican Simeon Fess by a wide margin (1,276,208 to 839,068) and served one term in the Senate from 1935 until 1941, before retiring. In 1940 Democrats in Ohio asked him to consider running for president as a favorite son in an effort to aid Franklin D. Roosevelt's bid for a third term, but he declined. (The plan would have had Ohio's delegates to the Democratic National Convention pledged to Donahey until Roosevelt became a candidate, at which point Donahey would release the delegates to Roosevelt.)