(((He has endeavoured to prevent the population of these States; for that purpose obstructing the Laws for Naturalization of Foreigners . . . He has obstructed the Administration of Justice, by refusing his Assent to Laws for establishing Judiciary powers. He has made Judges dependent on his Will alone, for the tenure of their offices, and the amount and payment of their salaries.)))

Q. How is parallelism used here to strengthen the argument?

ACapitalizing certain words helps the reader skim the document and arrive at the conclusion more quickly.
BMixing up the length of the lines suggests how random the King's poor actions were.
CRepeating "He has" followed by a negative noun highlights that the king did one bad thing after another.
DUsing short words adds a sense of urgency and nervousness to the argument.

Parallelism or parallel structure in grammar could be defined as two or more components in a sentence or text that have the same grammatical structure for the sake of impressing the reader. This method also adds balance and rhythm to sentences.

In this excerpt, parallelism is used in the following way:

C. Repeating "He has" followed by a negative noun highlights that the king did one bad thing after another.

He has followed by a noun is a structure repeated in each sentence, so as to emphasize each of the King's failures.