The U.S. Senate will begin debate next week on the so-called fast-track trade legislation that is key to conclude the Trans-Pacific Partnership (TPP) negotiations, Senate Majority Leader Mitch McConnell said Thursday.

Senators will vote on a cloture motion scheduled for next Tuesday to proceed to the fast-track legislation, which would allow the upper chamber to formally begin its floor debate, said the top Republican in the Senate.

The procedure vote will be an initial test for the Senate's support for the trade legislation, as 60 votes will be needed to clear the hurdle in the 100-member chamber with 54 Republicans usually in favor of free trade.

Liberal Democrats and activists from labor unions have launched campaigns to oppose the fast-track legislation, arguing that trade deals have hurt U.S. workers and increased income inequality.

Senate Democratic leader Harry Reid has also threatened to hold up the legislation until the Senate moves forward on two other bills related to highway construction funding and domestic surveillance program.

The Senate Finance Committee and House Ways and Means Committee had approved the fast-track legislation last month, but final congressional passage is far from certain.

The fast-track legislation, formally known as trade promotion authority (TPA), empowers the president to negotiate trade deals and then present them to Congress for up-or-down votes, with no amendments allowed.

If the legislation is passed by Congress, it will give U.S. trading partners the confidence they need to put their best offers on the table and help the Obama administration conclude the ongoing TPP free trade talks, which are nearing completion after more than five years of negotiations.

"Our trade negotiators cannot bring this Pacific agreement back to Congress for careful review and deliberation unless Congress assures our trading partners that the agreement is going to get a fair up or down vote," McConnell said, urging senators to back the trade legislation.