In British history the Whigs were generally the political party which supported the interests dissenters such as the Puritans and the early industrialists against the aristocracy.  In the American Colonies, the Whigs  supported the interests of the growing revoltionary movement while the opposing Tories supported the policies of the British.  After the Boston Tea Party (which happens in chapter seven of Johnny Tremain) the British close the port at Boston and basically impose martial law on the city.  Anyone who had known Whig leanings would be suspected as a revolutionary associated with Hancock, Adams, and the Tea Party, and so would not be considered welcome by the British in Boston.