The polls show too many voters find the quiet man easy to ignore

Iain Duncan Smith's immediate political problem can be simply stated: too many of his MPs have developed doubts about his political skills and judgement as party leader.

But what underscores those doubts? The question usually resolves itself, one way or another, into whether Mr Duncan Smith has the ability ever to make an impact on the public. The evidence from the polls is not encouraging.

It is bad enough that among those voters who have formed a judgement about the Tory leader, an apparently growing number – even amongst Conservative supporters – say that he is not doing a good job. What is even worse is the large proportion who still do not have a view about him at all.

According to the latest Mori poll, 38 per cent still have no opinion about how well the Leader of the Opposition is doing. No other leader of the opposition has evinced so little reaction after a year in the job.

Unlike MPs, the public care little about performances in the House of Commons. If they did care, William Hague would have secured higher ratings than Tony Blair. Their judgements are based on what they see on television.

And Mr Duncan Smith's problem is that he lacks presence in the studio. He is not a Jeremy Paxman, a Peter Snow or a David Dimbleby who can impose and convey their personality on the screen. Seemingly a little stiff and a little reserved, he is the quiet man who apparently can be, and consequently is, ignored. Of course effective television is not just made in the studio. Fiery rhetoric can fill the voter's living room too. But Mr Duncan Smith is no orator either.

But what if Tory MPs were to depose Mr Duncan Smith? Are any of the pretenders capable of commanding the attention and respect of voters?

Undoubtedly the one potential candidate who does have that ability is Kenneth Clarke. He remains by far the most popular potential Tory leader amongst the public as whole.

In a YouGov poll taken at the end of last week, no less than 36 per cent said that Mr Clarke would be the best leader of the Conservative Party, well ahead of the 13 per cent who named Mr Duncan Smith. In contrast only 5 per cent backed his most likely rival, David Davis. Replacing Mr Duncan Smith with Mr Davis would be to replace an unknown with an unknown.

However, changing leaders will not on its own immediately improve the party's fortunes. YouGov's poll suggests that while the immediate impact of making Mr Clarke leader would be to cut Labour's lead by three points, Mr Blair would still be six points ahead. But such a boost might mean no more talk of the Liberal Democrats replacing the Conservatives as the main opposition.

The danger of that happening should not be exaggerated. YouGov's latest poll still puts the Conservatives 10 points ahead of the Liberal Democrats. And as things currently stand at least, the Liberal Democrats would have to be well ahead of the Conservatives in votes before they came ahead in seats.

Moreover, at recent elections most voters have felt the Liberal Democrats were far closer to Labour than to the Conservatives on the big issues of the day such as tax and spend. That would have to change before many true-blue Tories could come to regard the Liberal Democrats as an acceptable alternative home.

But Tory MPs would be wrong to believe that disaster could never happen After all it already has in Scotland. There the party has not only lost the mantle of chief opposition to the SNP but now trails the Liberal Democrats as well.

Changing leader once more would undoubtedly be an embarrassment. But if the alternative is endless squabbling that could indeed be tantamount to signing the party's death warrant.

John Curtice is professor of politics at Strathclyde University.