Legislation giving President Barack Obama leeway to negotiate free-trade agreements faced its first hurdle in the US Senate Wednesday, with lawmakers divided over what negotiating demands should be included.

Members of the Senate Finance Committee launched into a marathon debate ahead of an expected vote by the panel on a bill that would give the president "fast-track" authority on a Pacific accord deemed the largest free-trade agreement ever.

Most Republicans are in favor, while some Democrats are bucking their president and expressing vehement opposition.

The debate comes as US officials negotiate the framework for the Trans-Pacific Partnership (TPP), an ambitious trade pact with Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam.

If it passes the full Senate and House of Representatives, the law would create a simplified mechanism for Congress to approve or reject any agreement negotiated by Obama, while preventing lawmakers from making changes -- a procedure that has been used in the past to avoid congressional stalemates.

Orrin Hatch, who chairs the Senate Finance Committee, pleaded with colleagues not to block the "fast-track" legislation.

Without it, he said, "our trading partners will not put their best offers on the table because they will have no guarantees that the agreement they reach will be the one that Congress votes on in the end."

Supporters insist the TPP will level the playing field for American workers, and allow Washington to help write the rules of the 21st century global economy rather than its economic rival China.

Critics argue that the TPP maximizes corporate profits, while killing American jobs and dismantling labor, environmental and safety standards.

Lawmakers inserted nearly 150 negotiating demands on the administration in their bill, mainly in the fields of intellectual property, labor law and the environment.

Senator Ron Wyden, the measure's chief Democratic co-author, said the promotion of human rights will be made a formal part of the negotiation objectives for the first time.

Lawmakers have long complained that trade pacts are negotiated in absolute secrecy.

But the bill would require the White House to publicize the final TPP text 60 days before it is signed.

Some on the Democratic left, allied with labor unions, are hostile to the pact, concerned that opening the US market will weaken America's already declining manufacturing sector.

Asked Tuesday if he would support TPP, Senate Minority Leader Harry Reid had a feisty retort: "Hell no."

Republicans too have expressed concern, particularly about the lack of enforceable currency provisions in the fast-track bill.

"If we can't get a good deal on currency, I'm no," Senator Lindsey Graham told reporters, adding that several of his Republican colleagues had similar reservations.

Obama pushed back against the hostility and promoted TPP, which could affect nearly 40 percent of global GDP.

"When you hear folks make a lot of suggestions about how bad this trade deal is, when you dig into the facts they are wrong," Obama told MSNBC Tuesday.