WASHINGTON (Reuters) - The Senate approved a sweeping Wall Street reform bill on Thursday night, capping months of wrangling over the biggest overhaul of financial regulation since the 1930s.

By a vote of 59 to 39, the Senate awarded a victory to President Barack Obama, a champion of tighter rules for banks and capital markets after a 2007-2009 financial crisis that slammed the economy and led to massive taxpayer bailouts.

The Senate bill must now be merged with a measure approved in December by the U.S. House of Representatives. Only then could a final package go to Obama to be signed into law, something that analysts said may happen next month.

Changes proposed in both bills -- driven by lawmakers eager to look tough on Wall Street ahead of mid-term congressional elections in November -- threaten to constrain the banking industry and reduce its profits for years to come.

Obama said the final version of the bill will hold financial firms accountable but not stifle the free market.

“Over the last year, the financial industry has repeatedly tried to end this reform with hordes of lobbyists and millions of dollars in ads, and when they couldn’t kill it they tried to water it down .... Today, I think it’s fair to say these efforts have failed,” Obama said.

“We’ve still go some work to do,” he added. “The House and the Senate will have to iron out the differences between the two bills. And there’s no doubt that during that time the financial industry and their lobbyists will keep on fighting.”

On Wall Street on Thursday, the Dow Jones industrial average slid 3.6 percent, hurt by fears of Europe’s debt crisis retarding global economic recovery, but also by uncertainty over U.S. financial reform, traders said.

Barney Frank, head of a key House panel, told CNBC it was important to complete reform soon to ease uncertainty. He said Obama could sign a bill into law well before July 4.

FRANK DRAWS EARLY LINE

Frank, the Democratic author of Wall Street reform in the House, on Thursday drew an early negotiating line ahead of impending talks with the Senate on a final package.

In letters to senior Senate Democrats that were obtained by Reuters, Frank said certain House proposals on financial firm regulation and bank trading limits must be preserved.

He said the House proposals were important to his home state of Massachusetts and that “none of them threaten or weaken the broad objectives of comprehensive reform ... I will insist that they be maintained in the final bill.”

The letters were addressed to Senate Democratic Leader Harry Reid and Banking Committee Chairman Christopher Dodd, the Democratic author of the Senate bill. Both will likely be key players, along with Frank, in the House-Senate talks.

Last-minute maneuvering on the Senate floor killed two controversial amendments: one to tighten proposed restrictions on risky trading by banks, and another exempting car dealers that do not finance their own lending to auto buyers from oversight by a new federal consumer watchdog.

Republicans withdrew the auto-dealer amendment, offered by Senator Sam Brownback, so that the bank trading amendment, offered by Democrats Jeff Merkley and Carl Levin, would not come to a vote. It is firmly opposed by major financial firms.

‘POWER OF WALL STREET’

Traders work on the floor of the New York Stock Exchange, May 20, 2010. REUTERS/Brendan McDermid

Expecting such a move, Levin told reporters beforehand that it showed “the power of Wall Street” at work in Congress.

The Merkley-Levin amendment would have tightened language in the Senate bill on the “Volcker rule” proposed in January by Obama and White House economic adviser Paul Volcker. As it stands now, the bill contains the rule, but leaves it up to regulators to write the details and possibly water it down.

Levin and Merkley said they would push in the House-Senate conference for tightening the Volcker rule provision.

The House bill already contains a watchdog exemption for auto dealers, opening the door to a deal in conference.

As approved, the Senate bill also contains a provision from Democratic Senator Blanche Lincoln that would force banks to spin off their lucrative swap trading desks into affiliates.

Major financial groups such as JPMorgan Chase, Bank of America and Goldman Sachs could be hit hard by such a requirement, analysts said.

Sheila Bair, chairman of the Federal Deposit Insurance Corp, reiterated concerns that Lincoln’s approach “could increase, not decrease, risk.” Analysts said it was unlikely to survive the conference. It is not in the House bill.