By Nick Assinder BBC News Online political correspondent

Every time Iain Duncan Smith slaps down one assault on his leadership, another one pops up somewhere else.

Actually, it is always the same plot but dressed up in different clothes and being wielded by a different character.

Last week it was former Tory chairman Michael "Tarzan" Heseltine insisting the party had to dump its leader and replace him with the so-called dream ticket of Kenneth Clarke and Michael Portillo - whatever the grassroots members wanted.

Now the Sun newspaper has dusted off its "dead parrot" - first deployed against previous Tory leader William Hague - to declare Mr Duncan Smith is running out of time.

At the same time there are suggestions that euro enthusiast Clarke is ready to launch a challenge if the government rules out a referendum on joining the single currency this parliament.

Quiet man

And there are persistent reports that, if the Tories don't do well in next May's local elections, a challenge will be inevitable.

It doesn't take a genius to work out that something is up - and it might just be Mr Duncan Smith's time.

It was pretty easy for the self-proclaimed quiet man to brush aside Hezza's assault.

Clarke still tipped for job

This is, after all, the man many Tories see as a serial assassin who did for their beloved Margaret Thatcher and has continued in his chosen trade ever since.

But the latest rumblings are a bit more substantial.

They point to the prospect of a leadership challenge next summer and there are, indeed, two events which could give the plotters their excuse.

Poor showing

If Chancellor Gordon Brown's decision on the euro really is a "no" then it would effectively kill off that as an issue until beyond the next poll, making his candidacy less troublesome with the largely Eurosceptic Tory MPs.

And if the party fails to pick up large numbers of local council seats in May - and that probably means triple figures - things will look even more grim than they currently do.

Indeed, there are even some Tory backbenchers privately suggesting the best thing that could happen to the party is a poor showing in May so they have the excuse they are looking for.

All this comes against the background of the Tories' appalling poll showings.

Despite the government's troubles on several fronts, Mr Duncan Smith is seen by even those close to him to have failed to lay any decent blows on Tony Blair.

So once again, the swirl around the House of Commons is that unless he gets some signs of progress quickly, his time may indeed be up.