The increasingly widespread - and perfectly legal - practice of gerrymandering is having a serious and lasting effect on American democracy, as the BBC's James Silver reports from Texas.

The sight of cowboys being abruptly unseated by the bucking broncos at the rodeo in Mesquite, Texas, must surely strike a chord with many of the state's Democratic Congressmen.

Thanks to gerrymandering - the redrawing of district boundaries for partisan advantage - several of them are set to lose their seats in November.

Using sophisticated computer programmes, political boundaries are being redrawn across the US to all but guarantee results.

Gerrymandering is quietly, but dramatically, changing the face of American democracy.

Today, only a handful of Congressional seats are truly competitive.

Unwinnable district

Simply by redrawing the lines of his district with the click of a mouse, the Republicans - who hold power in the state of Texas - have forced Democratic Congressman Jim Turner into a district he knows he can't possibly win.

That's why, come November, he will be standing down.

"The redistricting lines which were drawn by the Republicans in Texas were clearly aimed at trying to eliminate as many Democrats as possible from the state's delegation to Congress," he told BBC Radio 4.

"I happened to be the member that was targeted most effectively, because my district was cut into six different pieces and the part that I live in was put into a district that's been represented by a Republican congressman for almost 20 years.

"This is a dangerous trend. We know democracy is not promoted if we end up with partisan politicians selecting their constituents rather than the other way around."

The term gerrymandering can be traced back to 1812, when Massachusetts Governor Elbridge Gerry fashioned a district for political purposes in the shape of a salamander.

It has been part of the political landscape in America ever since.

Every decade, with the publication of a new census, all US states are obliged to redraw the boundaries of their congressional districts.

With a handful of exceptions, the process is carried out by the party in control of the state legislature.

Deadlock

In Texas, there was deadlock between the parties in 2001 - the regular year for redistricting - so the job was handed to the courts.

They drew boundaries which by and large preserved the congressional majority of the Democrats.

However, when the Republicans gained complete control of the state of Texas in 2002 - for the first time in 130 years - they took the unprecedented decision to return to the redistricting table mid-decade.

Jim Turner may be standing down, but five of the seven Democrats affected by the new map have decided to fight on.

They include Congressman Martin Frost, who has represented district 24 of Dallas-Fort Worth for 26 years.

Congressman Frost claimed that the Republicans "were trying to turn Texas into a one-party state".

But, he continued, the shadow of gerrymandering falls on far more than just the state of Texas - it falls across national government too.

"Gerrymandering can be used by one party or the other to preserve their majority in the House of Representatives long past the time they have a majority in the country.

"It can also alter the course of legislation which can cause the Congress to move in a direction which is contrary to majority sentiment in the country."

Forced out of his own district by the new plan, Mr Frost is now running in a Republican-leaning district containing far fewer natural supporters.