I'm suffering from scandal fatigue.

You wonder: is there anyone in Westminster who isn't "at it"?

Hardly had we come to terms with the extraordinary tale of the suspended Asian Tory candidate who had allegedly enlisted the English Defence League to stage-manage a race march, than Channel 4 Dispatches and the Daily Telegraph revealed yet another haul from their latest cash-for-access sting operation.

A businessman, Peter Wilmott, posing as a high-rolling party donor, had been sent off by the programme to see if he could gain privileged access to the upper levels of all three main parties: Labour, the Conservatives and the Liberal Democrats.

Needless to say, after waving a wad of cash, he was wafted graciously into the inner sanctums of the political establishment. Vigorous handshakes were exchanged with party leaders, as well as other stuff.

Yes, this is the same Channel 4 Dispatches that secretly recorded Malcolm Rifkind and Jack Straw apparently offering to work as highly paid consultants for a phoney Chinese business group only last month. Has there ever been an easier way to generate front-page scoops?

A LibDem peer, Lord Strasburger, has already resigned after allegedly telling Channel 4's under-cover man how to get round the rules on party donations. This was barely a week after a LibDem candidate, Ibrahim Taguri, had resigned over an earlier donations sting.

In both cases, the issue was manipulation of the rules to prevent those actually making donations having their real names registered. That was coupled with suggestions - clear but rarely explicit - that "doors would open" if they put the money up front, though whose doors isn't clear.

And just in case Labour gets all smug and self-righteous, Dispatches also revealed one of their shadow ministers, Chris Leslie, allegedly discussing policies that could further his business interests. And hardly a day goes by without another Ukip figure going "rogue".

In the interests of balance, I'm wondering if the Scottish National Party should have a scandal assigned to it if it can't come up with one of its own. Is the SNP inherently less corruptible than the other parties? I can't think of an obvious reason why.

It has its own big business donors such as Sir Brian Souter, chairman of Stagecoach. Former First Minister Alex Salmond has been known to share Tunnocks with Rupert Murdoch and there was that episode with Donald Trump and a golf course that didn't end well.

But the SNP don't seem to get involved in actual dodgy dealings with these people.

Perhaps the fact that the SNP don't take up seats in the House of Lords makes it less vulnerable. Perhaps it's just that no one thinks it is worth buying.

Regardless of the reason, let's hope that their membership, now standing at 100,000 (an extraordinary achievement in a cynical age) keeps the party leaders on the straight and narrow - and away from businessmen bearing gifts. After all, why do we think they're prepared to hand huge sums of money to politicians? They aren't fools; businessmen use money to make money. When they hand tens and even hundreds of thousands of pounds over to a political party, they naturally expect something in return.

More often than not, it is some peerage or gong. Last week, researchers at Oxford University confirmed that "the relationship between party donations and nominations [to the House of Lords] has been found to be significant". This is similar to saying that there is a significant correlation between the Pope and the Catholic faith.

Sometimes it looks like vanity: paying big money to rub shoulders with Prime Minister David Cameron, for instance. But donors also know that just being "on the radar" of people in power can lead to advantage in future.

Back in the 1990s, this used to be called sleaze - something disreputable if not exactly illegal. The term largely fell into disuse because everyone thought parliament had cleaned up its act. Clearly there is work still to do.

But these omni-sleaze revelations are extraordinarily damaging to the big parties in the run-up to the General Election.

Each front-page headline about a dodgy donation or a cash-for-access sting just makes people less prepared to vote for the Conservatives, Labour or the LibDems.

If the SNP do actually win a landslide in May, disgust with Westminster's politicians-for-sale culture might well have played a significant role.