There were two British political groups (like today America has Republicans and Democrats). Parliament was split into the Whigs and the Tories.
In America, though they didn't have representatives in Parliament, people still split into one of the two groups when it came to their political leanings. Whigs tended to support the crown and Parliament, through all the restrictive legislation that was passed. Whigs tended to be more sympathetic to the plight of the American colonies, and more resistant to the new taxes, etc, that were being passed.
The American Revolution was sprung out of the Whig Party. Alan Warren is a fictitious character, but his "cousin", Joseph Warren, was a real man, and one of the most well known American leaders of the Whig party and was involved in all sorts of government affairs in Massachusetts.
Hope that answers that!