By Mark Davies BBC News Online political reporter

Duncan Smith says he will focus on Labour failures

Take, for instance, the Tory leader's interview on Radio 4's Today programme on Thursday.

Such was his determination to fend off questions about internal divisions with jocularity that his repeated chuckling prompted a flurry of bemused press comment on Friday.

The problem for Mr Duncan Smith is that he can laugh it off all he likes, but this is the story that won't go away.

This week has seen a flurry of reports about splits at Tory Central Office following the departure of officials seen as party modernisers.

In short, the argument goes, Mr Duncan Smith is clearing out those who favour reform of the party and a more socially liberal approach to policy.

Instead he is drafting in traditionalists and, rather like William Hague before him, appears to be returning to tread a more right-wing path after an early dalliance with a more "inclusive" agenda.

Forced

Tory chairman Theresa May, a key moderniser, was said to be furious and considering her position - though she denied such claims. Right-wingers like John Redwood welcomed the changes.

Mrs May denied that talks were being held with members of the Conservative Party's board over the changes - and was later forced to admit that some talks had taken place.

And all this after Mr Duncan Smith's desperate appeal last year to the party to "unite or die".

Michael Portillo rules himself out of running for the leadership

But for all the apparent disarray, there was one key factor missing. If this really was as severe a division as it sounded, where were the voices protesting at the shake-up?

Certainly, there were plenty of anonymous comments from the "insiders" who always lurk on the edges of these affairs. But hard and fast on the record comment was hard to find.

No longer. Michael Portillo remains one of the most influential of Tory voices. And the sheer strength of his comments sets the story alight.

Battle

Why? Because Mr Portillo is speaking for a substantial element in his party which is dismayed by Mr Duncan Smith's leadership.

They highlight that what is happening among Conservatives is, in essence, a battle for the heart and soul of the Tory Party. Where a few months ago, the modernisers seemed to hold sway, now the traditionalists have a spring in their step.

The latter feel also that when voters go to the polls in May, Mr Duncan Smith could be buoyed by a backlash against Labour's National Insurance increase, higher council tax bills and those opposed to Tony Blair's stance on Iraq.

A positive showing in May would give the Tory leader breathing space - but some of the modernisers fear that under his leadership electoral annihilation will follow at the next general election if Mr Duncan Smith is still at the helm.

What Mr Portillo is doing is essentially casting huge doubts on the Tory leader's ability to hold his party together.

Devastating

He won't challenge him for the leadership, he says - but there will be those out there who may be ready to take up the challenge.

Or in other words, by breaking cover and wielding the knife Mr Portillo could clear the way for his followers to make further more devastating attacks in the future.

All this is happening in a recess of course - so MPs will return next week in what, if those other voices emerge, could become a fevered atmosphere in the Commons.

And if more senior voices come out to back Mr Portillo's view, it will mean very serious trouble indeed for the Tory leader.