Someone who knows he is too small to change politics, but still wants to fight

Solution 1:

I think the artist you are referring to can be described as a Don Quixote:

• An impractical idealist bent on righting incorrigible wrongs. (After Don Quixote, hero of a satirical chivalric romance by Miguel de Cervantes.)

Quixotic:

• Caught up in the romance of noble deeds and the pursuit of unreachable goals; idealistic without regard to practicality.

(AHD)

Solution 2:

Pragmatic describes the willingness to do something in spite of the disappointing expectations:

adjective

1 Dealing with things sensibly and realistically in a way that is based on practical rather than theoretical considerations:
a pragmatic approach to politics
_ODO

Cynic describes that sense of hopelessness:

1.1 A person who questions whether something will happen or whether it is worthwhile:
the cynics were silenced when the factory opened
_ODO

Should this person be called a cynic or a pragmatist? It is hard to choose between the two, but since this is a single word request, pragmatic seems to embrace the notion of cynicism sufficiently. Pragmatists understand the limits of a situation and simply do what they can to make the most of it.

Together, the two words make a great team. For his noble efforts to keep the ethnic tinderbox of Europe at peace in the early 20th century, the statesman Klemens von Metternich was honored by Arnold Blumberg with the label of pragmatic cynic:

The splintering of Central Europe, and the creation of so many nonviable states since the war, may give us renewed respect for the pragmatic cynic whose name is forever associated with the years 1815-1848.
_{Great Leaders, Great Tyrants?: Contemporary Views of World Rulers who Made History}

As Plato is said to have said:

The penalty good men pay for indifference to public affairs is to be ruled by evil men. _{Congressional Record, Volume 98, Part 6, Page 7944}

John Stuart Mill concurred:

Let not any one pacify his conscience by the delusion that he can do no harm if he takes no part, and forms no opinion. Bad men need nothing more to compass their ends, than that good men should look on and do nothing.
_{Rectorial Address Delivered at the University of St. Andrews}

Solution 3:

Political Underdog may best represent someone who knows they have little chance of winning yet takes on the challenge with a glimmer of hope rather than delusion. There are numerous articles using this term covering individuals to minority parties.

Solution 4:

There are several phrases that I might give, but few single words. I hope that one of them might suit the needs of the OP.

If someone is showing dogged, naive determination in the face of political opposition, he might be referred to Mr. Smith, from the movie, Mr. Smith Goes to Washington. In this 1939 Frank Capra movie, the title character played by Jimmie Stewart fights political corruption because of his conviction and personality. (However, I think current audiences would more readily go to a character in The Matrix.)

A word for such a fighter might be naive.

If someone is showing noble determination but is unaware of the true nature of the political opposition, he might be referred to as Quixotic, as mentioned by Josh61. The task that the person is involved in may be called Tilting at Windmills.

The task that someone is engaged in might be called a fool's errand which the Free dictionary calls

an attempt to do something that has no chance of success

A single word for this might be foolhardy.

Someone who is fighting against a faceless bureaucracy and failing may be told, You can't fight city hall.