First U.S Senator elected by the populace who was a serious candidate for President in party nom

[Benjamin Frank]

I don't know the answer to this.  U.S Senators were not elected in all states by the public until 1914 and the first U.S Senator after this to be elected President was not until 1946 with Harry Truman who became President as the Vice President, and the first U.S Senator to be directly elected President was not until John Kennedy in 1960.

But, who was the first popularly elected Senator to be a serious candidate for a Presidential nomination?  

I am aware that the Presidential nomination process did not open up to voters in general until 1972 and, obviously George McGovern was the first Senator nominated under that process (though ironically he did so by dominating the less democratic caucus states and ended up with far less primary votes than former Vice President Hubert Humphrey.)

A little hard to believe that since U.S Senators were elected by the populace that only two of them: John Kennedy and Barrack Obama have been directly elected President.

[Independents for Nihilism]

Warren G. Harding (remember him? can't blame you if you don't) was elected Senator in 1914 and served (in the loosest sense of the word) until he was elected President.

[Benjamin Frank]

Warren G. Harding (remember him? can't blame you if you don't) was elected Senator in 1914 and served (in the loosest sense of the word) until he was elected President.

Ah, thanks. I always thought he was a Governor not a Senator.  He was the President of whom it was said that he 'looked like a President.'

[Mr. Smith]

Warren G. Harding (remember him? can't blame you if you don't) was elected Senator in 1914 and served (in the loosest sense of the word) until he was elected President.

Ah, thanks. I always thought he was a Governor not a Senator.  He was the President of whom it was said that he 'looked like a President.'

Ironically he beat the Governor of his own state, and even a relatively popular Governor at that too.