In an appointment that the White House described as routine, Noel Hillman, in charge of unravelling the burgeoning bribery scandal on Capitol Hill that could ensnare dozens of Republican congressmen and their staff, will step down next week after accepting the offer of a judgeship in New Jersey.

A group of Democrats demanded that a special prosecutor take over the investigation. The furore comes a month after Mr Abramoff, once one of the most powerful lobbyists in Washington, pleaded guilty to bribing politicians.

He is expected to co-operate with prosecutors and implicate up to half a dozen congressman, in a case managed for two years by Mr Hillman, who has intimate knowledge of its details.

The latest twist in the Abramoff case, a politically explosive issue ahead of November’s mid-term congressional elections, comes as Mr Bush faces mounting pressure to release at least six photographs of him meeting Mr Abramoff at White House parties and receptions.

Referring to Mr Hillman’s judgeship offer, George Miller, a Democrat congressman from California, said: “It looks like the White House has reached in and tampered with an ongoing investigation.” Calling the timing of the offer startling, he said: “You have one of the chief prosecutors removed from a case that has tentacles throughout the Republican leadership of Congress, throughout the various agencies and into the White House.”

Chuck Schumer and Ken Salazar, two Democrat senators, wrote to Alberto Gonzales, the Attorney-General, calling for a special prosecutor to take over to “ensure that the investigation and prosecution will proceed without fear or favour”. The White House said that Mr Bush was poised to nominate Mr Hillman last summer, but did not say why it was delayed until this week.

Scott McClellan, Mr Bush’s spokesman, said the calls for a special prosecutor were “pure politics”, adding that prosecutors at the Justice Department “are continuing to fully investigate” the Abramoff case.

Mr Bush continues to be dogged about the photographs. Mr McClellan conceded that Mr Abramoff, who lavished millions of dollars in campaign contributions on dozens of congressmen from both parties, had attended “staff-level” meetings in the Bush White House.

But he said that Mr Bush “does not recall meeting Abramoff”.

Democrats argue that the photographs were taken at taxpayers’ expense and that in 1997 Bill Clinton was forced to release videotapes of him meeting big donors at White House functions.

This month Mr Bush’s re-election campaign dumped $6,000 it had received from Mr Abramoff, his wife and clients, giving the money to charity.

Many other politicians have also given Abramoff-linked donations away. Asked on Thursday why he would not release the photographs, Mr Bush said: “I’ve had my picture taken with a lot of people. Having my picture taken with someone doesn’t mean that I’m a friend with them or know them well.

“I’m also mindful that we live in a world in which those pictures will be used for pure political purposes, and they’re not relevant to the investigation.”

Although there is no suggestion that Mr Bush had any improper dealings with Mr Abramoff, a report this month stated that Mr Abramoff displayed on his office desk at least five pictures of himself and Mr Bush.

He told people that when they met, the President talked about his childrens’ names and discussed personal details about their schooling.

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