U.S. Secretary of State John Kerry, center left, greets Boeing employees prior to speaking about the Trans-Pacific Partnership during a trade speech at the Boeing's 737 airplane factory in Renton, Wash., on May 19. (Saul Loeb/AP)

The Senate headed toward another critical hurdle in President Obama’s bid to secure a sweeping Pacific-rim trade deal on which he has staked much of the prestige of the presidency.

With a key vote slated for Thursday, the Obama administration and Senate Republicans faced a new threat from Democratic demands to extend the Export-Import Bank, whose charter for helping U.S. corporations sell goods abroad expires June 30. The agency has drawn fire from conservative critics who view it as a form of corporate welfare, and leading Republicans running for the 2016 presidential nomination have disavowed an entity that most big business supports.

In addition, White House officials worked with Republicans to try to thwart the latest proposal to create a tougher international system for cracking down on nations that manipulate their currency to make their exports cheaper in global markets.

Supporters of the Trade Promotion Authority — the legislation that would set the stage for a trade deal with 11 other nations — remained hopeful that they could hold together their coalition of more than 60 Republicans and Democrats, but they recognized that several previous supporters were now holding out.

“We’re going to grind on and finish TPA, if those who say they’re for it end up voting for it,” Senate Majority Leader Mitch McConnell (R-Ky.) said in a brief interview.

If they clear Thursday’s filibuster hurdle, supporters expect final approval to come by the weekend, which would then send the TPA bill over to the House, where it faces a more perilous path to passage because of entrenched opposition from many House Democrats.

The president has said that he needs this fast-track authority to complete the deal with the other nations, representing about 40 percent of the global economy. If approved by Congress, it would allow for a trade deal to be presented to the House and Senate under strict timelines and require a simple up-or-down vote without any amendments or requirements of a Senate super-majority to end debate.

Obama, who considers his trade agenda a key legacy item of his final years in office, has engaged in a personal form of persuasion with lawmakers that he has previously avoided. Last week, he held a two-hour meeting in the Cabinet Room with 10 Democrats, and on Tuesday another group of lawmakers headed to the White House for a lengthy meeting with the president.

Sen. Benjamin L. Cardin (D-Md.), whose support wavered briefly over procedural matters last week, said he has never received such attention from the president on any issue.

“Believe me, I’ve got scars,” he said Wednesday.

Losing Thursday’s procedural vote would be a stunning blow to Obama, who would see for the second time in two weeks a successful filibuster by his own party of his trade agenda. After last week’s temporary blockade, 13 Democrats joined with almost every Republican to begin debate on the legislation.

A second filibuster would, at a minimum, force McConnell to pull the trade legislation until early next month as the Senate must take up legislation involving expiring portions of the 2001 Patriot Act.

By late Wednesday, several prior supporters of the legislation said they expected to join most of the Democratic caucus in supporting a filibuster because there had been no headway made on the Ex-Im Bank.

“The Freedom Caucus seems to still control the issue,” said Sen. Maria Cantwell (D-Wash.), the biggest supporter of the bank. That’s a reference to the most conservative caucus in the House, which has had its sights set on the Ex-Im Bank for several years and now is just weeks away from seeing its ability to extend new loans freeze.

Three years ago, more than 300 House members, including 149 Republicans, supported extending the bank’s charter, but the ranks of its opponents have grown beyond just the most conservative Republicans. Rep. Paul Ryan (R-Wis.), the party’s 2012 vice presidential nominee and chairman of the Ways and Means Committee, opposes Ex-Im, as does House Majority Leader Kevin McCarthy (R-Calif.) and House Majority Whip Steve Scalise (R-La.). McConnell has said he opposes it but would allow a vote on the floor because a clear majority of senators appear to support the bank.

Sen. Patty Murray (D-Wash.), an influential member of Democratic leadership who supports the trade bill, said she supports Cantwell’s position and would block TPA unless she was assured Ex-Im had “a path” to getting reauthorized.

The Washington Democrats have a Republican ally, Sen. Lindsey Graham (S.C.), who is one of the few 2016 contenders proudly supporting the bank. He vowed in a floor speech Tuesday night to block the trade bill, known as voting against cloture, until he got a deal after months of private negotiations on Ex-Im.

“I’m tired of talking,” Graham said. “You’re not going to get my vote for cloture or anything else this year until I get a vote, we get a vote on the Ex-Im bank.”

All three senators represent states where Boeing, a top Ex-Im client, is a major employer.

Another tricky maneuver is fending off the latest currency amendment, which was drafted by Sens. Rob Portman (R-Ohio) and Debbie Stabenow (D-Mich.), a bipartisan pair from Midwestern states whose manufacturing industries have been ravaged by foreign competition.

[Read more about tough reelections for Portman and other ‘Rust Belt Republicans.’]

Previous bids to advance tougher language on currency were sidestepped by creating a separate piece of legislation that won wide approval with 78 votes last week, but now Portman — who faces a difficult reelection in 2016 — has maneuvered this amendment into a position in which it would be part of the fast-track legislation and it only requires a simple-majority vote to succeed.

“I don’t think this is a radical concept at all,” Portman told reporters this week.

On Tuesday, Treasury Secretary Jack Lew wrote to Senate leaders that he would recommend vetoing the bipartisan proposal because the other nations party to the Trans-Pacific Partnership would back out of the trade deal.

A majority of Republicans are expected to instead support a softer alternative drafted by Sen. Orrin Hatch (R-Utah), chairman of the Senate Finance Committee, along with his Democratic ranking member, Sen. Ron Wyden (D-Ore.), and a few other Democrats supporting the TPA bill.

So far, four Republicans are supporting the Portman-Stabenow amendment, which is expected to receive strong support from the 46 members of the Democratic caucus. That sets up a close-call vote that is likely to come Thursday afternoon.

“We can win. That’s why the White House is so concerned,” Sen. Sherrod Brown (D-Ohio), the leading opponent of the trade legislation, told reporters Wednesday.

Mike DeBonis contributed to this report.