* JPMorgan chief to appear before Senate on June 13
* Likely to be pressed on how much trading losses have grown
* Testimony seen as appetizer before second-quarter results
* Dimon previously was known as Washington's banker friend
By Dave Clarke and David Henry
WASHINGTON/NEW YORK, June 10 (Reuters) - Jamie Dimon will be playing a new role in Washington this Wednesday, called to explain JPMorgan's recent trading debacle after years of being known as the Wall Street banker who got it right during the financial crisis.
The Senate Banking Committee is expected to press Dimon on how much more will the estimated $2 billion trading loss grow, and whether the purported failed hedging strategy was really a speculative bet that went largely undetected until it was too late.
The embarrassing loss from a series of trades out of JPMorgan's London office has also raised questions about the oversight of regulators and whether proposed rules curbing proprietary trading will be adequate.
The Senate Banking Committee has asked Dimon to come prepared to provide "a thorough accounting of the trading losses," a committee aide said. Senators will also ask what he knew about the risks involved in the trading strategy.
Analysts say Dimon, who was once Washington's token banker ally after JPMorgan salvaged the wreckage of Bear Stearns and Washington Mutual during the financial crisis, can't be too coy in his answers.
"The main question on people's minds is how did this happen?" said analyst Jason Goldberg of Barclays. "People don't understand how something can go from nothing to something in relatively short order and not be detected until it was too late."
So far Dimon has been contrite, calling the trades "sloppy" and saying "egregious mistakes" were made.
But in four prior public appearances about the trades, Dimon declined to provide many details on what happened, saying that he feared doing so would give trading adversaries clues to how to take advantage of JPMorgan's still-open positions.
Mark Calabria, a former Republican aide on the Banking Committee, now with the libertarian Cato Institute, said Dimon will at least have to show he's got a handle on the portfolio.
"He's got to relay that 'I've got control of the company, I've got some sense of what's going on and there are not a whole lot of little bombs in the company that I'm not aware of,'" Calabria said.
Goldberg from Barclays expects the market will get just a morsel of information about the trades. Dimon has said that investors and analysts will have to wait for more details on the portfolio until mid-July when the company announces second-quarter financial results.
"I view Wednesday as the appetizer, but you have to wait to mid-July for the main course," said Goldberg.
Dimon also may shed more light on the bank's decision to radically change the way risk was measured in the Chief Investment Office responsible for the loss.
WHALE OF A LOSS
Dimon initially pegged the loss at $2 billion on May 10 when he announced the derivatives losses generated from the bank's London office and trader Bruno Iksil, dubbed the "London Whale" in credit markets due to the size of the trading positions he took.
At the time, Dimon said the loss could go to $3 billion, "or more." Some analysts have estimated the losses could reach $5 billion, based on market talk about the exact trades.
Even at $5 billion, the loss would not be debilitating for the company, which last year spent $3.2 billion on litigation and still made a $19 billion profit.
The loss has raised larger questions about whether bank executives and regulators can spot growing risks before it's too late.
It also has weakened Dimon's position as the unofficial spokesman for Wall Street banks as they push for more moderate versions of reforms called for in the 2010 Dodd-Frank financial oversight law.
Democrats are expected to press Dimon on whether the losses could have been prevented by the trading restrictions in the so-called Volcker rule and whether he now considers his criticism of this policy as misguided.
Democrat Sherrod Brown will quiz the CEO on whether his bank and others like it are too big to manage, according to a spokeswoman.
Richard Shelby, the top Republican on the panel, plans to ask Dimon "why he is so adamantly opposed to the primary measure that would protect taxpayers against further bailouts - higher capital requirements," the senator's spokesman said.
Shelby and other Republicans have criticized many Dodd-Frank reforms as too complicated to work and have portrayed higher capital standards as a more elegant answer to the question of how to make sure large banks can absorb losses.
On June 7, the Federal Reserve released a set of proposals to implement new Basel III capital standards agreed to by the heads of the world's 20 leading economies.
Dimon has been critical of the agreement, arguing that parts of it, such as how mortgage servicing rights are treated, are unfair to American lenders and ultimately will lead to fewer loans being provided to individuals and businesses.
READY TO RUMBLE
Profiles of the often combative Dimon have noted he took up boxing after leaving Citigroup Inc in 1998 during an executive shakeup.
For him personally the Senate hearing, and another hearing planned by a House of Representatives panel for later this month, will be about more than showing he can take a political punch -- they are opportunities to restore a diminished reputation.
"It's the first step in what will be a long road to restoring his credibility politically," said Brian Gardner, an analyst at Keefe, Bruyette & Woods Inc. (Reporting By Dave Clarke and David Henry; Editing by Karey Wutkowski and Tim Dobbyn)