[jjotto]

I believe only a couple of states did not already have popular election of senators when the 17th was introduced.

Note the 1858 Lincoln-Douglas debates with the candidates playing to the crowd.

[j.havenfarm]

Lincoln Douglas has always been interesting. Playing to the crowd, yes, but for the purpose of getting them to influence the Legislature, which still did the voting

[Pelham]

Dunno if wiki has it right but:

At the time, U.S. senators were elected by state legislatures; thus Lincoln and Douglas were trying for their respective parties to win control of the Illinois General Assembly.