However not a conjunction

"However" is not a conjunction.

 The word may not be used to link two clauses in a sentence (a clause being a potentially independent sentence with its own subject and verb). The conjunctions are for, and, nor, but, or, yet and so.

If you want to use "however",  you must first stop the previous sentence with a period or a semicolon (or, rarely, a colon).

The following sentence is therefore wrong:

Partisanship has always been a part of politics, however, polarization has caused the divide between parties to widen too far.

The "however" is being misused to join the first part (clause)  of the sentence (Partisanship has always been a part of politics) with the second (polarization has caused the divide between parties to widen too far). What you need instead is a simple "but" or "yet" preceded by a comma to link the clauses thus: 

Partisanship has always been a part of politics, but/yet polarization has caused the divide between parties to widen too far.

Alternatively, you should bring the first part of the sentence to a punctuated end before using "however", as follows:
Partisanship has always been a part of politics; however, polarization has caused the divide between parties to widen too far.

or:

Partisanship has always been a part of politics. However, polarization has caused the divide between parties to widen too far. 

or even (my favorite):

Partisanship has always been a part of politics.  Polarization, however,  has caused the divide between parties to widen too far.