In politics, hardline refers to the doctrine, policy, and posturing of a government or political body as being absolutist and sometimes authoritarian. The hardline position is often extremist and uncompromising.
The term hardliner can be synonymous with the term "hawk" and is often used in the pejorative sense. It can be used to refer to individuals within a political, special interest, or activist movement of almost any ideology.
A hardliner is someone who sticks to his point rigorously and does not adopt differing ideas quickly, or at all.
References
Hardline Wikipedia(Text) CC BY-SA