The independent clause in this sentence is Many voters registered for the election.
An independent clause or main clause is a clause that can stand by itself as a simple sentence. An independent clause contains a subject and a predicate and makes sense by itself, that is, it conveys a complete thought.
In this sentence, after the independent clause, a dependent clause follows which is introduced by "but."
Dependant clauses can be joined by using a semicolon or by using a comma followed by a coordinating conjunction (and, but, for, or, nor, so, yet, etc.).