US President Barack Obama on Tuesday hailed a "breakthrough" in his laborious effort to pass sweeping Wall Street reform, after Democrats apparently lined up the votes to pass the landmark bill.

Obama praised three Republican senators for reaching across party lines in the bitterly-divided Senate to give the Democratic leadership sufficient support to advance the measure, his top current priority.

Earlier, a holdout Democratic Senator Ben Nelson said he would vote for the bill, leaving only one Democratic member of the chamber, Russ Feingold from Wisconsin, opposing the measure.

Republicans Olympia Snowe and Susan Collins of Maine and Scott Brown of Massachusetts, had earlier signaled their support and barring last minute switches, should give Democrats the 60 votes they need to overcome Republican blocking tactics.

"Three Republican senators have put politics and partisanship aside to support this reform, and I am grateful for their decision as well as all the Democrats who have worked so hard to make their reform a reality," Obama said.

"What members of both parties realise is that we can't allow a financial crisis like this one that we just went through to happen again.

"This reform will prevent that from happening," Obama said at the White House, as he unveiled the nomination of Jacob Lew to serve as his budget director.

"I urge the Senate to act quickly so that I can sign it into law next week."

The measure would enact the biggest overhaul of Wall Street rules since the Great Depression of the 1930s, and give regulators new weapons against risky investment practices blamed for the 2007-2009 global financial meltdown.

It creates a new consumer financial protection agency, an early-warning system to predict and prevent the next crisis, and mechanisms aimed at liquidating rather than saving companies once deemed "too big to fail."

The legislation also closes loopholes in regulations and requires greater transparency and accountability for hedge funds, mortgage brokers and payday lenders, and arcane financial instruments called derivatives.