The Chief Secretary to the Treasury says: "Of course, there is a vast amount more to do but, frankly, if you remember back to 2010, the mess Labour left us in, the huge risks there were for our country, it's only because the Liberal Democrats stood up and took responsibility for helping to see the country through these difficult times that we are able to now look ahead with a bit more optimism and confidence."
It is clear Mr Alexander believes if the Tories had been left to their own devices, Britain's economic recovery would not have been assured. "We have played an essential role in seeing this country through our darkest economic hour and enabling a recovery to go ahead.
"Remember back to 2010 and the mess our country was in. We faced a difficult choice: Do we go into coalition with the Conservatives or do we stand back? If we had stood back, it would have been a calamity for this country. And not only joining the Government, but then shaping the economic policy of the Government to make sure it responds in the right way and in a pragmatic way to the economic conditions that we faced. So, yeah, we have been essential, absolutely."
Within the corridors of power, one source suggested the mood had, of late, lightened, even describing the deputy Chancellor as perky.
Mr Alexander laughs: "I'm positive about the future. I've always been someone who has been..." Perky? "Not my word but maybe. Optimistic."
He added: "We have worked bloody hard to see through some very hard things that needed to be done. But we are not in any way taking our foot off the pedal and stepping back from things that need to be done, so I'm not sure perky is the right word."
The Chief Secretary estimates Britain is "halfway" through the austerity programme and the cuts needed to bring the mammoth deficit down. He says there is "absolutely no complacency" in any part of government - despite what Vince Cable, the Business Secretary, might say - and carefully describes how "it's early stages, but we are into the recovery phase".
Interestingly, he detects the lighter mood apparently pervading his department is now also stretching slowly across the country. "For the first time for a while people can look to the future and see some brighter prospects ahead, but there is a long way to go to entrench the recovery."
As for Labour, Mr Alexander is scathing, saying Ed Miliband's Plan B has not died because "I don't think it was ever alive".
Labour, he claims, made a "catastrophic mistake" by not reflecting on the mess it had made of the economy, apologise for it and change its approach. He says Labour's strategy is now "falling apart. In pretty short order, they have to grow up on the economy or they're going to have a real problem at the next election.
"The Liberal Democrats are the only party that can build a stronger economy in a fairer society, enabling everyone to get on in life. If you want to keep the economy strong, you can't trust Labour to do that, if you want society to be fair, you can't trust the Conservatives to do that. Therefore, you need to have the Liberal Democrats at the table if you care about both those things."
l The deputy leader of the Scottish Liberal Democrats will today launch an attack on a UK Government campaign to encourage immigrants to leave Britain.
Alistair Carmichael will tell LibDem colleagues gathering for their conference in Glasgow that controversial UK Border Agency posters were "disrespectful".
The posters, which have been trialled at the UKBA office in Glasgow, carry the message: "Let us help you go home." Addressing a rally at the Scottish Exhibition and Conference Centre to mark the start of the LibDem conference, Mr Carmichael will say: "Crass, aren't they? Insulting. Disrespectul. A disgrace."