Nick Clegg was in the Today programme hot seat this morning, being grilled on the coalition Budget's "progressive" credentials in a 15-minute interview with the veteran BBC attack-dog John Humphrys.
Humphrys taxed Clegg with "the most significant reversal of the welfare state since World War II", and asked whether when he took over the leadership of the Liberal Democrats in 2007 he thought he would find himself co-operating to push through a Budget that "would hit the poor harder than the rich".
Clegg did his best to refute Humphrys, saying that the measures announced in the Budget would in fact oblige the top 10 per cent to make eight times the cash contribution than the lowest-paid.
But he also said that he preferred not disappear into the "undergrowth of claims and counterclaims about the statistics of the Budget", only to be rebuked by Humphrys, who pointed out that "we can't lightly dismiss all the statistics, because they are what it's about in the end".
This opening exchange set the tone for the entire interview, with Clegg always trying to move away from details of the impact of specific measures into well-rehearsed generalities about "fairness", "difficult decisions" and the "inherited mess".
When Humphrys challenged the Deputy Prime Minister with the statement  from the director of the Institute for Fiscal Studies (IFS) describing this Budget, minus the measures inherited from the Labour goverment, as "regressive", Clegg floundered, attempting to say that the report did not take into account "any attempts we might make to instil fairness in future budgets", and could thus be discounted.
Gone was the smooth orator who instigated "Cleggmania" after the first televised leaders' debate. Instead, we heard a harassed-sounding Deputy PM, who failed to defend the measures of his goverment and quibbled over the semantics of the critical IFS report.
Every time Clegg appeared to approach a valid point, as in the case of the coalition raising the minimum threshold for paying tax, Humphrys was ready with an awkward fact that made him seem out of touch with the real message. In attempting to pass off the freezing of child tax credits as "difficult decisions I wish we didn't have to take", Humphrys was ready with the retort "it isn't just about difficult decisions, it's about things you said you wouldn't do".
VAT was another big stumbling block for Clegg, as Humphrys pointed to the campaign poster  that the Lib Dems used to attack the Tories over the issue, as well as the Lib Dem deputy leader Simon Hughes's statement  of his own opposition to the "most regressive" tax. Clegg's response, that "no party in the general election campaign ruled out that we might have to raise VAT", was greeted with derision from Humphrys, who exclaimed, "That is disingenuous!" only to receive an incoherent rebuttal from the under-pressure Clegg.
In extremis, Clegg resorted to the old trick of blaming the coalition's predecessors for the measures in the austerity Budget, only to slip up again and say: "If we were to sit on our hands as the Labour government is, sorry the Labour Party is doing . . ."
Finally, confronted with Richard Grayson's recent remark  that the Liberal Democrats are now "a centre-left party that is being led from the centre-right", Clegg responded:
I am a Liberal politician to my fingertips . . . and I think there's something morally wrong with sitting on our hands and risking a double-dip recession.
It was the only moment in the entire interview when Clegg managed to turn an answer round to make his own point, something at which he has previously shown himself to be very adept.
The impression listeners will have taken from this interview is of a politician under strain, and failing to address the fundamental question couched so succinctly by Humphrys:
Why should the poorest 10 per cent pay anything to get us out of this mess? They're the poorest.