Sen. Jeff Sessions, R-Ala., called on GOP lawmakers to block a "Fast Track" trade bill on Tuesday that President Obama hopes to finalize in order to secure a string of new trade pacts.

Sessions, one of the Senate's staunchest opponents of the Trade Promotion Authority legislation, or TPA, said new trade deals threaten to cause further economic damage and job loss in the United States and could lead to a back-door deal with China later on.

Approval of TPA will give President Obama expedited power to secure new trade deals, including the Trans-Pacific Partnership with 11 Pacific Rim nations, or TPP. But Sessions said in his letter that Obama has not spelled out how the U.S. economy might change if the TPP were approved, and that Obama is looking to inject environmental issues into the agreement.

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"All of this information gives us more than enough basis to slow down and not fast track anything until all of our questions are answered," Sessions said in a letter sent to Republican Senate lawmakers on Monday. "We should be inherently skeptical of grand designs, too complex to oversee, whose creators can provide no specifics yet pledge utopian results."

Sessions' call to oppose the bill came just two hours after Senate Majority Leader Mitch McConnell signaled he was optimistic lawmakers will approve a two-part trade package this week.

The plan calls for lawmakers to vote on TPA on Tuesday, and then vote on Wednesday to extend the Trade Adjustment Assistance Act, a retraining and aid program for workers displaced by trade deals.

Democrats in the Senate said they will not vote for TPA unless the worker retraining bill is signed at the same time by President Obama.

"It was always the goal to ensure these bills passed Congress in the end," McConnell said. "It remains the bipartisan goal today. We're now on the verge of achieving it. With just a little more trust, a little more cooperation, and simply voting consistently, we'll get there."

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The Senate approved the legislation last month but is taking it up as two different bills after House Democrats sunk the package by voting down the worker aid legislation.

Many Democrats and Republicans remain opposed to the deal, so votes in the Senate this week will likely be close.

Sessions argued in the memo that there remain too many outstanding questions about the upcoming trade deals that will be expedited under the TPA bill. He pointed out that Obama has acknowledged China may seek to join the Trans-Pacific Partnership in the future, after Congress votes up or down on the pact.

Lawmakers have been angered by the secrecy surrounding the deals and how they might change after becoming law and without further oversight by Congress.

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China is accused of currency manipulation, which has hurt the U.S. economy and has made it harder to export American products.

"Conservatives demand caution," Sessions said in the memo. "Can any of us honestly say to our constituents we understand the full implication of the … agreements that will be pre-approved for adoption?

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