Although deputies stress that there is no suggestion of any move against Ahern, they admit that there are "rumblings about Bertie" and that "tolerance levels" towards the Taoiseach have dropped.

These rumblings have intensified in the wake of Friday's embarrassing opinion poll in The Irish Times which showed the Taoiseach's popularity plummeting and, crucially, Tanaiste Brian Cowen having a higher personal approval rating.

"This is the first time ever that the idea is being challenged that he's so popular that he's untouchable, " said one TD.

There remain many in the Fianna Fail parliamentary party who believe Ahern will go at a time of his own choosing and possibly not until after the local elections in 2009.

However, this is no longer the unanimously-held view it was a couple of months ago. A number of TDs now believe the changeover could happen much sooner and that it might not prove as seamless as had previously been expected.

"I think his number is up, " said one Fianna Fail source. "If we fall further in the polls, fellas will get anxious and you will see stalking horses emerge. There's nothing to focus minds like opinion polls".

"The Christmas period could be interesting if things don't improve from this poll.

You asked me about Bertie's future, it's the polls that will provide the answer: it will depend on the polls between now and Easter, " said another TD. He added that if pushed on the question, he believed Ahern would be gone as leader before the summer.

This was echoed by another TD who said that the Taoiseach would be "ok" for "as long as he's a vote-getter", adding that he believed he would be gone in the next year or so, because "his day is done".

Even those who insist that Friday's poll does not bring forward the Taoiseach's departure accept that "the time is coming".

While Fianna Fail TDs insist that the issue of the Taoiseach's finances is not coming up with constituents, many accept that his credibility has been seriously damaged by his spell in the witness box in Dublin Castle, and subsequent polls showing voters were unhappy with his testimony. There is a view that the possibility of the Mahon Tribunal's final report being published next year is likely to bring forward Ahern's planned date of departure.

There is also irritation at the Taoiseach's handling of the recent controversy over increased pay for ministers.

TDs privately believe Ahern's initial response to the criticism was "arrogant" and "too smart". One TD told the Sunday Tribune that "he wouldn't be doing those things if he was going to be there long-term."

It is widely accepted that none of the main contenders to succeed Ahern will move against him, but some of Cowen's admirers are said to be getting itchy feet. There is some concern that if Ahern's popularity continues to drop, it will also affect senior figures who are forced to defend him.