Although it's dark outside, and having been up until 1am the night before – after two days of meetings with the Treasury, the Federal Reserve and her own oversight committee – the woman charged with overseeing the Treasury's $700bn (£471bn) bank bail-out fund is far from tired.
Having spent the majority of her working life focusing on middle-class families and bankruptcies – her day job is as a law professor at Harvard Law School, where she has taught for the last 18 years – it's little surprise that she's looking out for middle America in her temporary role to point out the good and bad of the Troubled Asset Relief Programme (TARP).
"We can't have a robust economy when we're looking at 1m foreclosures," she says. "Our economy just can't take this." Citing statistics that show almost one in four US mortgages is under water, she notes that government attempts to help those in trouble have failed, saying that for every borrower who has avoided foreclosure, another 10 lost their homes.
"Today we've got a [housing] market that won't heal itself, and if it doesn't heal, we'll all be paying this year, next year, and for years to come."
Prof Warren points out that the original idea behind the bail-out fund was to buy parcels of toxic mortgages burning holes in banks' balance sheets: "Part of what we're trying to accomplish is to remind Treasury that it should be about mortgage foreclosures, that's the trigger for the crisis that has not yet been resolved."
She blames the housing downturn on the very banking industry which was the recipient of half of the first wave of the $700bn fund, and "the bizarre way" many mortgages that are now delinquent were created, with "first mortgages on top of second mortgages" and individuals given home loans they could never afford.
Little surprise then that she is the most ardent supporter of a strong consumer financial protection agency as part of the current financial reform legislation winding its way through the US Senate.
Noting that there are currently seven US regulators who have some form of oversight over consumers – the largest being the Fed – she wants an independent agency to look out for middle America.
"I want Senators to have to vote 'yes' or 'no' on an agency with real muscle. I'm not interested in some pretend agency that lets everyone congratulate themselves when in fact it won't make any real difference."
At that point, Senator Richard Shelby, a leading Republican, walks by en route to his own interview and tells Warren: "You're doing good things."
It's ironic as the senator is blocking Democrat proposals for a consumer agency, saying it is "an incredible expansion of the government's reach without any basis in the current crisis".
Prof Warren knows she faces an uphill struggle, and admits that a year ago pushing through such a consumer body would have been easier: "The crisis had started with bad credit policies... I thought it would be the least contentious part of regulatory reform."
Part of the difficulty, she says, is that as banks' finances have recovered, and with foreclosures off the front pages, the problems of the past are further from the minds of those making the decisions. "When only the insiders are part of the conversation, then every policy that results is helpful only to insiders," she retorts.
A career academic, Prof Warren, 60, took up her current position in November 2008 when Senator Harry Reid, the Democrat leader in the Senate, asked her to chair the committee. A key part of her job has involved pointing out what she believes is wrong with not only the consumer agency proposals but other parts of the regulatory framework. On the need for a way to winding-up large institutions without wasting taxpayer funds, Prof Warren would like to see an authority that is "tough enough that it truly liquidates these financial institutions in an orderly way".
She adds: "They need to understand the consequence of their mistakes is that the shareholders are wiped out, the top management is fired and creditors take a haircut."
She is equally unrelenting when it comes to derivatives legislation, speaking of a "huge shadow market" born out of investment banks' realisation that there is little money to be made in equities because of greater efficiencies and transparency.
"The notion that a portion of our economy, which is so significant, can simply trade wildly with no transparency, with no clarity in how that market operates, is dangerous. If we didn't learn that over the last year-and-a-half, then we must have rocks for brains."
As she winds her way across Washington, stopping off for a photo shoot followed by a series of meetings, and talking to The Daily Telegraph along the way, she jokes that she's "a lot more fun in the classroom as nobody's taping" but it's clear she enjoys her role grand-standing on behalf of the American people.
That's why she taken the oversight panel on the road with stops in towns across America. "It's enough sometimes just to tear your heart out. The people who show up and want to talk always have something interesting, something personal, to say."
Prof Warren clearly has little time for the US capital's lobbyists and the banks they represent. "This is the new Wild West, the place where extraordinary profits can be made and the bonuses that come from them are to be sought," she says. "Remind me, what ARE they for? Business as usual? Because that did so well for us, right?"
Though effervescent and approachable, she dodges the question when asked if she would like to head the consumer protection agency – a role for which she is clearly well suited and often tipped for.
When pressed, she points out that she has spent her "whole career" on consumer protection and the middle classes, who "have [been] squeezed to the point that as a country we face the real possibility that our middle class could collapse."
In the next breath, she says: "You need a lot of things, and a leader is one of them. But I really don't want to make this about me."
But as a woman of the people – with a penchant for McDonald's morsels rather than Washington power breakfasts – she would say that, wouldn't she?
Professor Elizabeth Warren's CV
