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It's an unusual sort of speech.

This morning David Cameron says, in effect: "I'm in charge of the country. I have already promised radical welfare cuts which have yet to be implemented or examined for their impact but here's the sort of thing I'd want to do in three years time if you re-elect me without the need to rely on those pesky Liberals - but don't push me on the details yet."

So, why is he doing it?

There will be many theories, but here's just a few. It cheers up Tories in the week they're being asked to stomach Lib Dem plans to reform the Lords without a referendum; it gives Conservatives a way to highlight what they'd do if only they could; it helps get the Tory press back on side after the so-called omnishambles Budget and it poses real problems for Labour just as the housing benefit cap did.

There is, though, one reason above all others which the Westminster village too easily forgets and is the key to all the rest - it's popular.

A recent for Prospect magazine suggested a startling 74% of the public - including 59% of Labour voters and 51% of those on the lowest incomes (below £10,000) - thought that welfare payments should be cut. The most popular cuts are for, you guessed it, the unemployed and never-married single parents. The least popular cuts are for the elderly.

The prime minister is using his position to spark a debate which he already knows he's won. Asked about the specifics he will, no doubt, do as Iain Duncan Smith did this morning and say "these are the details... that's the challenge... we'll have to be careful ...etc"

One important note. David Cameron was careful to say - in the advance text we've seen so far - that he'd keep his promise to pensioners not to means-test their benefits, but Iain Duncan Smith made it absolutely clear that that promise lasts just for this Parliament - in other words not for the period the speech is talking about.