My Guess would be L.B. Johnson, who was a Naval Reserve Lt. Cmdr.
J.F. Kennedy served in the Navy but Congress came later. R.M. Nixon was a Naval officer. Ronald Reagan was in the USAAC (a captain). George H.W. Bush was in the Navy, but didn't enter until 1943. Jimmy Carter was Navy but WWII was over. Gerald Ford was Navy, but Congress came later.