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:
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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.
44
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.
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).