WASHINGTON - Debate over a huge international trade agreement has scrambled the normal political allegiances on Capitol Hill and across the Philadelphia region.

The city's three Democratic House members - usually reliable White House allies - have lined up against one of President Obama's top priorities.

Typical political foes such as Sen. Pat Toomey (R., Pa.) have sided with the president. Toomey voted to give Obama "fast-track" authority that could help him complete the Trans-Pacific Partnership.

The agreement would bind a dozen nations that account for 40 percent of the global economy, including Australia, Canada, Chile, Japan, and Mexico.

A final deal among the partners would still face congressional approval, but under the fast-track bill up for a House vote Friday, it could not be slowed by tinkering on Capitol Hill. That way, supporters say, negotiators are more likely to put their best offers on the table, free from worry that Congress will change the terms.

Obama and Republican leaders argue that the deal will open key markets to U.S. goods and boost the American economy. Critics say the deal will cost U.S. jobs.

The Senate in May passed the fast-track measure to help the deal along, but the House vote is expected to be excruciatingly close.

The issue isn't as simple as role reversal for area Republicans and Democrats.

Some Republicans close to labor unions - such as Rep. Frank LoBiondo of New Jersey - have joined union-friendly Democrats opposing the bill.

Another Republican with labor ties, Patrick Meehan of Delaware County, has backed GOP leadership in supporting the measure, despite stern warnings from the AFL-CIO.

New Jersey's two Democratic senators opposed the fast-track bill. Delaware's two Democratic senators supported it.

Labor unions have mounted an all-out campaign to stop the deal, winning over most Democrats and some conservative Republicans loath to hand Obama new power.

Trade votes often break down the usual right-vs.-left divide, said Josh Huder of Georgetown University's Government Affairs Institute.

"This is more of a district issue and a jobs issue - what kind of businesses are in your district?" Huder said. "How does trade affect your district?"

In places such as Philadelphia, where organized labor is strong, so is the opposition.

Philadelphia Rep. Robert Brady, a former Carpenters union leader and the city's Democratic chairman, is "100 percent" opposed to the bill, according to his office.

Rep. Brendan Boyle, another Philadelphia Democrat with labor backing, cited "21 years of trade deals where we can't point to one and say the average American worker is better off."

He and others point to the 1994 North American Free Trade Agreement, which they blame for stirring U.S. companies to export hundreds of thousands of jobs across the border, to where labor was cheaper.

"This is going down the same misguided path," said Rep. Donald Norcross (D., N.J.), former president of the Southern New Jersey AFL-CIO Central Labor Council.

Sen. Robert P. Casey (D., Pa.), one of Obama's closest friends on Capitol Hill, aggressively spoke against the bill, worrying that the deal would undercut wages.

AFL-CIO president Richard Trumka lobbied Democrats in Washington on Thursday. In Pennsylvania, the state chapter has held protests outside lawmakers' offices. Steelworkers marched through Pittsburgh in May holding "No Fast Track" signs.

"There are going to be some [angry] workers in Pennsylvania, and you can bet they're going to work hard to respond to people who tried to take food out of the mouths of their families," said Rick Bloomingdale, president of the AFL-CIO of Pennsylvania.

"I can't believe Patrick Meehan, the big oil refineries that are in his district, would even be considering voting for these agreements," Bloomingdale said.

Meehan said he expected he and labor would find common ground on other areas. "On balance," the trade deal "will actually allow us to continue to compete in a global economy," he said.

Meehan also faces pressure to support Republican leaders who gave him a prized seat this year on the powerful Ways and Means Committee.

The committee's chairman, former vice presidential nominee Paul Ryan (R., Wis.), is one of the most vocal supporters of the deal, as is Speaker John A. Boehner (R., Ohio).

"You don't get a spot on there if you're not going to do what the leadership wants you to do, when it comes down to it," Huder said. "I would be very surprised if any Republicans on Ways and Means voted against it. It would be a rebuke to leadership."

Republicans have formed an unusual alliance with Obama on the issue.

"There's more than a few items on which the president and I disagree, but when the president's right, I'll be happy to work with the president," Toomey said. "If we open up markets for American products, we will sell more American products. That will lead to more jobs."

Nearly $20 billion in goods from Pennsylvania and $13 billion from New Jersey were exported to Trans-Pacific Partnership countries in 2014, according to the Obama administration.

Like most House Republicans, Rep. Ryan Costello of Chester County supports the bill.

But supporters have been scrambling to find the final votes.

"I understand the concern of some in the labor community," Mayor Nutter, a close Obama ally, told reporters last month, "but I also know I've got a group of longshoremen who I'm sure would love to load some of our ships at the Philadelphia port and send those goods overseas."

Still, every Democrat representing the city is against the bill, including Rep. Chaka Fattah, despite Obama's pressing the Congressional Black Caucus for help.

At least two local Republicans are also defying their party leaders.

"This will further encourage U.S. companies to go offshore," LoBiondo recently told the Atlantic City-area radio station WPG.

A neighboring South Jersey Republican, Rep. Tom MacArthur, has vowed to support business-friendly policies but worries that Obama will agree to bad terms. "I don't think he's shown himself to be a very good negotiator," MacArthur said.

Bucks County Rep. Mike Fitzpatrick (R., Pa.) has expressed "concerns" about the deal.

He, MacArthur, and LoBiondo, however, cast votes Thursday that helped GOP leaders to a razor-thin procedural victory, setting up Friday's vote.