Senators Orrin Hatch and Ron Wyden, the chairman and ranking member of the Senate Finance Committee respectively, and Representative Paul Ryan, the chairman of the House Ways and Means Committee, reached an agreement Thursday on so-called “fast track” legislation that would ease the passage of President Barack Obama’s trade deals. The bill, known as Trade Promotion Authority (TPA), would prevent legislators from amending the trade agreements and limit Congress to an up-or-down vote on them. It’s an important milestone as Obama seeks to complete a twelve-country trade agreement called the Trans Pacific Partnership (TPP).

But fast track legislation has a long way to go before it becomes law. On Wednesday, hundreds of labor and environmental activists—including Senators Elizabeth Warren and Bernie Sanders, along with Representatives Rosa DeLauro, Keith Ellison, and Donna Edwards—gathered outside the Capitol in opposition to TPA. Soon after the rally ended, one of the biggest unions in the country, the United Food and Commercial Workers International Union, came out in opposition to the bill.

Legislators are understandably hesitant to pass TPA because it limits Congress’s role in the trade negotiations. But the White House’s position that TPA allows the United States to negotiate with one voice is also persuasive. In the end, the administration has a slightly stronger argument. But the key there is slightly. This is another example of how difficult it is for policy analysts and journalists to analyze trade deals.

What exactly is TPA, and why is the Obama administration so keen on passing it? TPA does two main things. First, it sets the negotiating objectives for the White House. In the version of TPA that Hatch, Wyden, and Ryan agreed on, those objectives require that the White House secure enforceable standards for labor, environment, and human rights, while demanding increased transparency, for instance. Through TPA, then, Congress is supposed to have a say in the actual negotiations. Equally important from the Democratic perspective, this TPA bill would last for at least three years, with a potential for a four-year extension. That means a hypothetical President Scott Walker would need to gain enforceable labor, environmental, and human rights standards in any trade deal his administration negotiates.

In return for setting those negotiating objectives, legislators agree to move the bill quickly through Congress. That means limited debate, no amendments, and just an up-or-down vote for passage. This bolsters the credibility of the president, allowing him (or her) to negotiate with other countries with the knowledge that Congress won’t adjust the deal after it’s completed. Either the legislative branch approves of what the president has negotiated and passes the deal, or it doesn’t and the deal dies.

“TPA makes clear to our trading partners that the Administration and Congress are on the same page in negotiating high standards in our trade agreements—standards that will protect our workers and environment,” said Andrew Bates, a spokesperson with the U.S. Trade Representative.

While Congress has passed TPA many times before, the legislation is more controversial this time. For one, the TPP deal is almost completed, meaning the administration negotiated the deal before Congress even set the negotiating principles. “The administration’s already said, ‘We met all your objectives,’ even though they haven’t been set,” Richard Trumka, the head of the AFL-CIO, which is planning a six-figure advertising campaign after TPA is introduced, said in March. “It’s a true joke. It makes a parody out of even fast-track.” Understanding this, Wyden negotiated a provision that allows Congress to revoke TPA if 60 senators believe that the president has ignored those negotiating objectives. That won’t be enough to persuade Trumka and other progressives to support fast-track. But in the end, the TPP will likely fulfill Congress’s objectives. If it doesn’t, Wyden has given Congress a method of recourse.

Many policymakers don’t just object to the timing of TPA, though. They object to the legislation altogether. “Do I look like a rubber stamp?” Representative Donna Edwards said at the rally Wednesday. “Fast track is a relinquishment of our constitutional responsibility,” Representative Alan Grayson told me after the rally. “We don't do this for tax bills. We don't do this for defense bills. We don't do this for health care bills. We didn't do this for agriculture bills. Why should we do it for trade bills?” There is, though, a key difference between trade bills and tax or health care bills: Trade bills are negotiations with many countries. Health care and tax bills are just domestic issues.

“Making law, deliberately in our Constitution, is one of the hardest things known to mankind,” Grayson added. “You have to have the House pass a bill and the Senate pass exactly the same bill, word for word, even down to the punctuation.” That’s true. But trade bills would become much harder to pass if foreign countries had to not just negotiate with the president but also all 535 members of Congress. Many of the partners to the TPP have been pressuring the U.S. to pass TPA.

At the rally Wednesday, multiple speakers referred to TPA as a “rubber stamp” on the president’s trade agenda. That’s a bit misleading. Congress will get a vote on the final trade agenda. If policymakers don’t like the bill, they can kill it.

It may seem strange that the AFL-CIO is already starting an ad blitz against the deal and that liberals such as Warren and Sanders are going to war over it. After all, TPA isn’t the actual trade deal. It makes more sense once you understand that many unions and progressives view this as the main fight. As Celeste Drake, a trade policy specialist at the AFL-CIO, said in an email:

No trade deal has ever been voted down under Fast Track. It’s misleading to argue that such a defeat would be a realistic possibility. By the time the deal is done, each and every Senator and Member of Congress is lobbied hard by whoever is in the White House, and by every single cabinet secretary. They are told if they vote no, they are turning their backs on our allies, diminishing the value of the word of the U.S. government, jeopardizing national security, etc. It is much harder to say NO to a done deal when there are only two choices and you lack an option to send it back to the table to meet the right standards. It’s better for Congress to have the leverage now over the contents of the agreement, to drag negotiations out until the contents of the deal are right, to do the messy work of improving trade policy at the negotiating table than to wait until everything comes up to one single yes or no vote and hope for the best.

If TPA passes, does that mean TPP will necessarily pass? I’m not convinced. The vote on fast-track is supposed to be extremely close, especially in the House. Once the final agreement is unveiled, some legislators who vote for fast-track may opt to oppose the deal. But I’m also sympathetic to Drake’s argument. Because TPA allows only an up-or-down vote on TPP, it means the trade bill can’t be filibustered, making it harder for opponents to block the deal.

The fast-track legislation will likely go through the respective House and Senate committees and onto the floor in the coming weeks. Labor groups are mounting an all-out assault on the bill.

“Make no mistake, we are determined to see this legislation defeated,” Marc Perrone, the president of the United Food and Commercial Workers International Union, said in a statement. “Our members will mobilize across the United States to call on Congress to stand up for hard-working families. While we may not be able to change every mind, our voices will be heard. And we will remember those who turned their back on America’s workers by voting for another destructive trade deal.”

This is a pivotal moment in Obama’s presidency. His trade deals are likely his last chance to move a big economic deal through Congress; the biggest opposition he'll face is from his own party. Warren, Sanders, and other progressives understand the stakes as well. As the legislation advances, their rhetoric will become stronger. The biggest intraparty fight of Obama’s presidency is officially underway.