In Ch. 20 Charlotte says of Willoughby "I do not think Mr Palmer would visit him, for he is in the Opposition, you know...". I think that here JA is using Charlotte to give the reader an important hint about what sort of man Willoughby is.

My knowledge of Georgian politics is rudimentary, to say the least and I'm happy to be corrected if I am mistaken, but here's my take on Charlotte's comment:-

From 1783 to 1830, except for a brief period of the 'Ministry of all the Talents' in 1806-7, the country was governed by solidly Tory administrations. Therefore "the Opposition" must refer to the Whig party.

From the late 17th century the Tory party stood for Crown privilege and represented the interests of the Clergy and smaller landowners. The Whigs represented great landowners and mercantile interests. After 1789, as a result of the French revolution and calls for constitutional reform, there was a realignment in British politics. The Tories took the 'conservative' position and stood for preservation of the existing order, whilst the Whigs favoured a reformist programme.

Crucially for JA's purposes, the Prince of Wales aligned himself with the Whigs, where he cultivated a licentious and spendthrift set of cronies. I believe that JA thought very poorly of the Prince of Wales, particularly for his amoral behaviour and the unkind, disrespectful way he treated his wife.