I've had a look at one-time/then-current Governors that were depicted on classic-era US commemorative coins and I've looked at former Mayors - this time let's have a look at US Senators that were depicted.
James Monroe (Rear Portrait)Before James Monroe became the 5th United States ("US") President (serving March 1817 to March 1825), he represented Virginia in the US Senate - he served from November 1790 to May 1794. He was depicted on the 1923
Monroe Doctrine Enunciation Half Dollar as the US President.

John Quincy Adams (Front Portrait)John Quincy Adams served as the US Secretary of State under James Monroe and followed him to the top spot, serving as the 6th US President (serving March 1825 to March 1829). Prior to this, however, he was a US Senator from Massachusetts from March 1803 to June 1808. He also served as a US Representative from Massachusetts in the House of Representatives from March 1831 to February 1848 (after his term as President).

William Bibb (Rear Portrait)William Wyatt Bibb, the first Governor of the State of Alabama (1819 to July 1820), represented Georgia in the US Senate from November 1813 to November 1816. Prior to his time in the US Senate, he represented Georgia as a member of the US House from January 1807 to November 1813.

Sam Houston] (Left Portrait)Sam Houston represented Texas in the US Senate from February 1846 to March 1859. Following his time in the US Senate, he was elected as the Governor of Texas and served from December 1859 to March 1861.

Carter GlassCarter Glass represented Virginia in the US House from November 1902 to December 1918. He was the US Secretary of the Treasury under Woodrow Wilson from December 1918 to February 1920 and was a US Senator from Virginia from February 1920 to May 1946 (i.e., until his death).
Joseph RobinsonJoseph Robinson represented Arkansas in the US House from March 1903 to January 1913 and briefly as the Governor of Arkansas from January 1913 to March 1913. He was elected to serve in the Senate by the Arkansas State Legislature to replace Jefferson Davis who had died just after the Legislature elected him to the Senate. Robinson resigned the Governorship and began his term in the Senate after jsut 55 days in office. He served in the US Senate from March 1913 until his death in July 14, 1937.

For other of my posts about commemorative coins and medals, including more Quick Bits stories and others about each of the coins referenced here, see:
Commems Collection.