Politics can certainly be dirty. For example, whenever there is an election coming up, candidates find all the dirt regarding their opponent. I guess to a certain extent it has to be this way in order to attempt to persuade people to vote in their favor.
Politics is dirty almost by definition. Notice when any candidate claims to want to run a "clean campaign" or wants to "take the high road" they get dragged into the gutter? In the 2004 Presidential campaign between John Kerry and George W. Bush, Kerry refused to aggressively counterattack charges that his service in Vietnam meant nothing, and that his wounds were fabricated as people at the Republican convention wore purple band-aids - this, as their own candidate, Bush, could not even prove his whereabouts in the Alabama Air National Guard at that time. Bush won the election. In 2008, McCain, who had been a class act throughout his career, pledged a clean campaign too, but when he fell behind in the polls and was attacked by the left, had to come out swinging in order to survive politically. Obama won the election.
Politics doesn't have to be dirty, but it almost always is, and not just in the US.
Of course it does all depend on your definition of dirty, but I think we actually have very obvious bribery and greediness that shows up all the time. Just a couple examples:
Remember the outrage over the warrantless wiretapping issue? Congress made a big show of opposing the bill that would give the companies retroactive immunity. This won them points with their voters. Not months afterwards, they all got some nice campaign contributions through the telecom companies and they quickly passed the bill quietly and without any real fanfare. Clearly a win win!
Obama's campaign was opposed to the idea of "drill baby drill" but as soon as he got in office his administration continued to grant environmental exceptions to oil companies to open up new areas for drilling. As long as it was done behind closed doors, no one knew, but he clearly couldn't make big oil angry, they have way too much money.
So maybe bribery only counts when it is in envelopes full of cash, but we have plenty of it and if that makes politics dirty, then yes, it is a dirty game.
I guess it is more or less dirty depending on what country you live in. Here in the US, it is dirty to some degree (depending on what you mean by dirty). But I think that this is inevitable in a democracy.
In democracy, we are always going to have politicians who need to pander to the special interests so that they can get money to run their reelection campaigns. But here, at least, I do not think that we really have clear bribery and that sort of corruption very often.