TAMPA, Florida (Reuters) - JPMorgan Chase & Co's (>> JPMorgan Chase & Co.) Jamie Dimon appears headed for a victory in a shareholder vote on whether he will keep his dual roles as chairman and chief executive of the largest U.S. bank, according to reports on Tuesday.

A bitter, months-long shareholder campaign demanding more oversight of Dimon will end with the vote tally at JPMorgan's annual meeting on Tuesday. Dimon had suggested that he may eventually leave the bank if he lost the vote.

The New York Times reported that JPMorgan appeared to have defeated the proposal to split the roles and create an independent chairmanship, citing people familiar with a preliminary vote count. The newspaper said the final outcome could still change and any margin of victory is still unclear. JPMorgan declined comment on the report.

Even so, Dimon was track to garner less support than he did in a similar vote at last year's meeting, when 40 percent of shareholders voted in favor of dividing the roles, Bloomberg News reported, citing two people with knowledge of the preliminary tallies.

Proponents of the independent chair proposal said if the measure gets 40 percent or more of the vote for a second consecutive year, the board should feel obligated to make at least some changes to increase its oversight of management.

Some investors said Dimon, 57, needs more oversight after the bank posted $6.2 billion in losses from failed derivative trades last year, but they do not want him to quit.

Among big-bank CEOs, Dimon ranks first for stock returns and has been praised for leading the bank through the financial crisis with no quarterly losses and a strong balance sheet.

If Dimon loses the vote and leaves the bank, the bank's shares could fall as much as 10 percent and erase about $20 billion in market value, according to Mike Mayo, a bank analyst with brokerage CLSA.

JPMorgan shares rose 1.3 percent to $52.95 on Tuesday.

JPMorgan also has no ready replacement for Dimon, Mayo wrote in a research note, adding that the two lieutenants best positioned to succeed him - Matt Zames, 42, and Mike Cavanagh, 47 - seem to be about three years short of being ready for the job.

Zames became sole chief operating officer of JPMorgan in April. Last year, Cavanagh became co-CEO of the company's reconstituted corporate and investment banking segment following a stint as head of treasury and securities services and several years as chief financial officer.

"Take a winning football team. One could always ask the question whether the team would have been as effective without the quarterback," said Benjamin Ram, a co-manager of the $1.6 billion Oppenheimer Main Street Select fund .

"The team gets part of the credit, but Jamie Dimon as the leader also gets the credit," Ram added.

Ram's fund has 6.4 percent of its assets in JPMorgan shares, more than any other diversified fund, according to Lipper, a Thomson Reuters company. He declined to comment on how he was voting.

The shareholder proposal is non-binding, meaning the bank's board does not have to follow the recommendation, even if the measure gets majority shareholder support. Still, a defeat would be a rebuke for Dimon.

A similar shareholder proposal last year won 40 percent of the vote, before most of the trading losses from the so-called "London Whale" imbroglio came to light.

JPMorgan's board recommended that shareholders vote against the proposal and the bank lobbied hard against the measure, with tensions rising in the run-up to the meeting.

Last week, the company that collects votes from investors, Broadridge Financial Solutions Inc (>> Broadridge Financial Solutions, Inc.), stopped telling shareholders how votes had been cast so far for the proposal and for other measures. Investors use the information to determine how to tailor their campaigns.

JPMorgan decided to release the results to shareholders after the New York Attorney General's office intervened over the weekend, a source familiar with the situation said on Monday.

"We were cut off from the tallies during the crucial week leading up to the meeting," said Dieter Waizenegger, executive director of the CtW Investment Group, which advises pensions that were voting against the bank in a separate measure regarding the re-election of directors.

Waizenegger said receiving the information at this late stage was of limited use.

The vote comes amid a growing trend in U.S. corporate governance to have an independent chairman lead the board. Many investors believe that doing so ensures the CEO does not have too much sway over the board and leads to better outcomes for shareholders overall. The debate, however, is far from settled.

Even if Dimon wins the vote, some shareholders plan to keep the pressure on the bank's board. Two major JPMorgan investors have told Reuters they will continue to press directors behind the scenes to increase their oversight over management.

One investor said they will likely encourage the bank to give more authority to lead independent director, former ExxonMobil (>> Exxon Mobil Corporation) CEO Lee Raymond.

(Additional reporting by Lauren Tara LaCapra; Editing by Dan Wilchins, Edwina Gibbs and Jeffrey Benkoe)

By David Henry