I think Obama was the only one to resign a senate seat to be president. Harding was in the senate during the campaign of 1920, but his term expired the same day as he took office as president. Truman has to resign a senate seat to become vice-president.
The following were U.S. Senators at the time of their election to the U.S. Presidency:
How many presidents had no military service at the time of their election?
Many US Senators have run for President without resigning their Senate seats. Those who have often run in their state's next US Senate election.
About one-third of the Senators in Congress are elected every two years. This means the 33 are up for election at any one time.
44
No US president was elected unanimously by popular vote. The only president elected unanimously by the electoral college was George Washington (There was no popular vote in this election).
How many presidents had no military service at the time of their election?
10
5
39 of the 44 U.S. Presidents to date have won at least one presidential election.35 of the 44 Presidents started their (first) presidencies by election; the other nine became president when their predecessors died or resigned.
Senators have 6-year terms and can run for re-election as many times as they want.
six
Tom Udall (D) and Martin Heinrich (D) are the U.S. Senators representing New Mexico. Udall's next election will be in 2020. Heinrich's next election will be in 2018.
No, though many presidents were previously governors (including Ronald Reagan, Bill Clinton, and George W. Bush). Many other presidents were senators or vice-presidents.
1/3 of the Senators' terms expire in every 2 years, but since there are 50 states with two Senators apiece and that does not divide by 3, in two election years 34 Senators are elected (or re-elected) and in the third year the remaining 32.
Claire McCaskill (D) and Roy Blunt (R) are the U.S. Senators representing Missouri. McCaskill's next election will be in 2018. Blunt's next election will be in 2022.
The progressive movement brought about a reform in the election of US senators. The reform provided for the direct election of senators into the legislature.
California has 2 U.S. Senators in Washington DC. Dianne Feinstein (D) and Kamala Harris (D) are the U.S. Senators representing California. Feinstein's next election will be in 2018. Harris' next election will be in 2022.