Usually you hear little about the quiet meetings, the lavish lunches and junkets that lubricate American politics. But every once in a while something comes along to open the system to what it hates most: daylight. The case of Jack Abramoff, influence-peddler extraordinaire, is one of those somethings.

Once Mr Abramoff claimed to have done nothing illegal, that his only sin was to have been too good at his job. But now his career is in ruins, a jail term of nine years or more beckons - an incarceration that would be even longer but for the plea bargain he reached yesterday with federal prosecutors.

For Mr Abramoff only contrition is left: "Words will not ever be able to express my sorrow and my profound regret for my actions and mistakes," he said in court yesterday. As for the two dozen members of Congress and their aides reputedly under investigation, they can only tremble.

If Mr Abramoff spills the beans, they may soon be contemplating a similar fate. This is potentially the biggest Congressional scandal of the modern era. It is largely (though not exclusively) Republican, and may mark the beginning of the end of the party's 11-year dominance of Capitol Hill.

Lobbying per se is nothing new. The right to "petition the government for a redress of grievances" is enshrined in the first amendment of the Constitution. Back in 1913, Woodrow Wilson said Washington was "swarming with lobbyists ... you can't throw a brick in any direction without hitting one".

But the 28th president cannot have imagined how access-peddling would blossom into a $4bn industry. There are 14,000 registered lobbyists, and as many again who are not registered. Between 1998 and 2004, foreign companies spent $620m (£350m) bending ears in Washington.

Lobbying thrives in the US for two reasons. In the US the executive and legislative branches are separate. The former is headed by the President, the latter consists of Congress, which writes laws and appropriates money for government spending. Although George Bush's Republicans have majorities in both House and Senate, he has no direct control of the bills they consider. That power rests with dozens of powerful committee chairmen and ranking members, all with their fiefdoms, whose yea or nay is decisive.

The other key ingredient is money, the colossal sums needed to fight election campaigns. In Britain, the curbs on such spending are strict. In America, by contrast, the sky's the limit. Total spending for the 2004 elections, presidential and congressional, reached $4bn.

The summit of extravagance was the 2004 Senate race in South Dakota, one of the least populous and less affluent US states. The two candidates spent a combined $40m. In an average state, the cost of defending a Senate seat is $20m. This means an incumbent has to raise $9,000 every day of his six-year term. At which point, enter the lobbyists.

The trade-off is simple. Corporate and other donors provide cash in a bid to secure the legislation they want. The intermediaries between the two sides are lobbyists. And the more people a lobbyist knows on Capitol Hill, the more effective he or she is.

Unsurprisingly, ever increasing numbers of them are former legislators. The Washington-based pressure group Centre for Public Integrity, says almost 250 former Congressmen and senior government officials are now active lobbyists.

Jack Abramoff and his ilk are key figures in Washington's power networks. And no network was mightier than the one embracing Mr Abramoff, the former House majority leader Tom DeLay and Grover Norquist, president of the arch-conservative Americans for Tax Reform, one of the most powerful special interests groups in Washington.