A critic in politics is one who will comment on events rather than participating in them. Criticism is always easier than making things happen: thus it is best to propose instead of oppose.
Many people are naturally critical of new or unfamiliar ideas. The tendency to criticize serves the function of being society's immune system against bad ideas.
Some critics are also opposed to participating in experiments or rigorous testing of new ideas, perhaps because they are traditionalists, or cynics, or pessimists.
Critics tend to slow down decision making processes, and lazy critics object to decisions only after they are made.
Critics are useful to prevent groupthink.
The civic best practice in dealing with critics is to ask them to define the positions they are in favor of, which in some cases are still the positions they are criticising.