At the time of the play there were two main political parties. The Liberals and the Conservative and Unionist Party. Broadly the liberals supported home rule for Ireland which was still a part of the UK and the Unionists opposed home rule. The liberal unionists had formed a smallish faction which changed its allegiance from liberal to conservative in the 1880s over the home rule issue. The point is that Jack is able to move easily within society as he has some common values with all, and also that he is not 'radical' and therefore is a safe person to invite into your social gatherings.