By Nick Assinder BBC News Online political correspondent

Tony Blair won't want to hear this - but there are more than a few of his MPs going around declaring Labour is now Gordon Brown's party.

As the prime minister and his chancellor present a united front in defending the tax-raising Budget, there is a growing feeling that theirs is a more unequal partnership than ever before.

Old Labour loves Brown

And it is Gordon Brown who is being seen as the man in the driving seat.

One backbencher, admittedly from the Old Labour wing, even went so far as to claim the chancellor had "given me back my party" with his tax and spend plans.

Blairites, understandably, have a slightly different perspective.

"Look," said one: "Tony is the leader. This is as much his budget as it is Gordon's."

Political battle

One or two, however, claim they are quite happy for Gordon Brown to take all the credit - because if it all goes horribly wrong, he will also take all the blame.

But it is impossible to see how the prime minister will be able to duck responsibility if voters reject the new direction.

Meanwhile, the more honest Tories are ready to admit the Budget has put Iain Duncan Smith firmly on the spot.

But they are also excited by the prospect of a real political battle ahead.

"This is very good news for us. We have a clear target and clear water between us and them," said one.

"But there is a problem - we've got to come up with an alternative that is convincing and will be effective," he said.

Real heat

It is certainly the case that, for the first time since the 1997 election, there is a clear, identifiable political gulf between the two parties.

Duncan Smith on the spot

And the shift of ground by the government has reinvigorated politics overnight.

This may not be the great ideological divide that marked the 1970s and 1980s - the chancellor has not thrown the New Labour baby out with the bath water.

But there is now some real heat in the discussions between the parties, not just inside the Commons chamber, but also around the Palace of Westminster in general.

And many Labour MPs - dismayed by the growing perception that there is nothing to chose between the two leading parties - hope this will spread into the general populace as well.

They are prepared to admit it is a risk and that they will probably lose some support as a result.

Leadership campaign

But they also believe traditional Labour supporters who have become disillusioned with New Labour's "small c conservatism" will be brought back into the fold.

Underlying much of this talk is the belief in the opposition parties and amongst some Labour backbenchers that the Budget represents a re-emergence of Old Labour.

They also believe the Budget has advanced Gordon Brown's campaign to succeed Tony Blair as leader and even prime minister.

If it all works then there is little doubt that the chancellor's standing will have been significantly enhanced.

It is also the case that tax and spend - and that is precisely what this Budget is about - was an Old Labour habit.

But much of this speculation has emerged only in the afterglow of the Budget.