Liberal Senate Democrats who until now were on the sidelines are complaining that a pending financial overhaul bill backed by President Barack Obama does not go far enough to rein in Wall Street's giants.

Some want the legislation changed to break up the nation's six biggest banks. Others would be content to restrict banks' ability to engage in speculative trading or to return to Depression-era rules that walled off Main Street commercial banks and their federally insured depositors from the lucrative investment houses of Wall Street.

To the chagrin of bankers and the White House, they could win some votes as the Senate this week begins considering amendments to a bill that Obama and lawmakers promise would prevent a repeat of the financial collapse two years ago, which set off the worst economic crisis since the Great Depression.

The six breakup targets are Bank of America, JPMorgan Chase, Citigroup, Wells Fargo, Goldman Sachs and Morgan Stanley. Together, they have assets that total more than 60 percent of the nation's gross domestic product.

"They're just too big," Sen. Ted Kaufman, D-Del., said in an interview. "They're too big to manage, they're too big to regulate."

The Obama administration, which has demanded tough new banking controls, worries that breaking up banks would hurt American competitiveness. The banking lobby, busy fighting some provisions already in the bill, fears a Senate majority will find populist appeal in cutting them down to size.

Kaufman and such Democratic senators as Carl Levin of Michigan, Sherrod Brown of Ohio, Jeff Merkley of Oregon, Sheldon Whitehouse of Rhode Island and Maria Cantwell of Washington, along with Independent Bernard Sanders of Vermont, plan to propose several changes.

Among the most outspoken on the need for a tougher bill is Kaufman, a lanky, professorial, 71-year-old former Senate aide appointed to the Delaware Senate seat of his former boss, Vice President Joe Biden.

Several times over the past three months Kaufman has gone to the Senate floor to make his case that the Senate's _ and by extension, the administration's _ regulatory remedy falls far short.

While Democratic Senate Banking Committee Chairman Christopher Dodd, D-Conn., sought to compromise with Republicans, Kaufman was criticizing the Democrats' regulatory scheme.

"Little in these reforms is really new, and nothing in these reforms will change the size of these mega-banks," he said in a floor speech a month ago.

Days later, ABC's "This Week" played a clip of Kaufman for guest Lawrence Summers, Obama's chief economic adviser. Soon after, Summers asked to meet with Kaufman. Summers' private message to Kaufman was not unlike what Summers would say publicly on PBS' "NewsHour":

"Most observers who study this believe that to try to break banks up into a lot of little pieces would hurt our ability to serve large companies and hurt the competitiveness of the United States. They believe that it would actually make us less stable, because the individual banks would be less diversified and, therefore, at greater risk of failing, because they wouldn't have profits in one area to turn to when a different area got in trouble."

Kaufman and his colleagues were undeterred by the White House's view. Last week, he and Brown filed an amendment to cut the size of a bank's permissible liabilities to no more than 2 percent of the gross domestic product and limit a bank's deposits to 10 percent of the nation's total deposits. The giant banks would have three or four years to wind themselves down.

"If we don't want more bailouts, we need to do something about the unprecedented concentration of wealth among a few large banks," Brown said.

Merkley and Levin propose banning commercial, or depository, banks from speculative trading with their own accounts. The amendment is tougher than Dodd's bill, which instructs an oversight council of regulators to study such a ban and recommend modifications and ways to implement it.

Merkley, visited by Treasury Secretary Timothy Geithner last week, says his proposal follows the spirit of Obama's recommendations. He added, though, "This may be a little clearer and crisper than the administration might have done."

Cantwell is considering offering an amendment based on legislation she and Republican Sen. John McCain of Arizona proposed in December. It would reinstate the 1930s Glass-Steagall Act that separated federally insured commercial banking from investment banking activities.

All those ideas could win support from some Republicans, many of whom have repeatedly maintained that no financial institution should be too big to fail. One of the intellectual forces behind the ideas is Simon Johnson, an economics professor at MIT. Variations on these and stronger measures also have been endorsed by Thomas Hoenig, president of the Federal Reserve Bank of Kansas City.

Dodd and Republican Sen. Bob Corker of Tennessee argue that size alone did not create banks too big to fail. They agree with the administration that the industry needs large institutions to compete in a global market. They note that of the top 10 banks in the world with assets of more than $2 trillion, only two are U.S. bank holding companies _ Bank of America and JP Morgan Chase _ and they don't rank in the top five.

Johnson counters that the Brown-Kaufman proposal would result in U.S. banks of $100 billion to $300 billion in total size. "Those banks would run rings around big, bloated, stupid banks in the rest of the world," he said.

For bank lobbyists, the amendments complicate an already difficult fight. They already take issue with some consumer protection provisions in the bill and a section that would force banks to spin off their lucrative derivatives business.

"Banning those companies from trading, hedge fund and private equity activity will only drive such activities into unregulated aspects of the financial system," said Rob Nichols, president and chief operating officer for the Financial Services Forum, which represents the nation's largest banks.