It wasn’t just Obama who lost Tuesday’s trade vote. It was Mitch McConnell.

On trade, the most important Democratic senator isn’t Elizabeth Warren. It’s Ron Wyden.

Wyden, a wonkish senator from Oregon, is the ranking member on the Senate Finance Committee, which has jurisdiction over trade issues. He’s also well-liked by more centrist Democrats, and given that the plausible coalition to pass the Trans-Pacific Partnership trade deal requires centrist Senate Democrats to partner with Senate Republicans, Wyden’s support is crucial for the administration’s hopes of getting the authority they need to finish TPP.

So far, Wyden has been a strong ally of the White House on the TPP — even though there’s been an anti-trade-deal blimp following him around when he goes home to Oregon. But on Tuesday the administration lost him, and a number of other pro-trade Democrats, on a crucial vote to move the package forward. The package failed.

I spoke to Wyden on Tuesday evening, and he made a point that’s often missed in coverage of the TPP fight. This isn’t just a negotiation between the White House and congressional Democrats. It’s also a negotiation between congressional Democrats and congressional Republicans — and in this case, it was a procedural decision by Senate Republicans that led to the Democratic defections.

To understand what happened in the vote, you need to know that there were actually four trade bills passed out of the Senate Finance Committee. Roughly speaking, there’s a bill on trade adjustment assistance for workers hurt by trade deals, a bill toughening enforcement provisions in trade deals, a customs bill that includes an extremely controversial provision on currency manipulation, and, crucially, a bill giving Obama the fast-track authoritynecessary to get the Trans-Pacific Partnership passed.

What happened in the Senate yesterday was that Democrats — and Wyden in particular — wanted all four bills considered, while Republicans only wanted to move forward on a smaller subset (namely, fast-track authority and trade assistance). Left out of the final package were provisions on trade enforcement and currency manipulation that are important to Democrats.

As is often the case, both sides think the other side broke its word on how the package would move forward, and the policy disagreements are complicated by procedural mistrust. Republicans see the Democratic demands as “yet another attempt by Democrats to dictate Senate Majority Leader Mitch McConnell’s agenda,” reports Politico.

But the bottom line is that this is a fairly typical fight between Senate Democrats and Senate Republicans that Obama finds himself on an unusual side of: he’s working with Senate Republicans rather than his own party. Obama typically negotiates with Republicans on behalf of packages put together by Democrats, but now he’s negotiating with Democrats on behalf of a package being crafted, in large part, by Senate Republicans. So when Obama launched an aggressive lobbying effort to try to get Senate Democrats — including Wyden — to vote for Tuesday’s package, it failed.

At the moment, this feels like a hiccup. Wyden says he and the pro-trade Democrats who defected yesterday still want to get the deal done. Harry Reid is floating the possibility of a compromise measure. The rift between Obama and Senate Democrats is, to some degree, obscuring a more traditional disagreement between Senate Democrats and Senate Republicans.

But it shows the tough position Obama is in on trade: he doesn’t have enough leverage over Senate Republicans to force a deal Senate Democrats will like, and, given how opposed Democratic interest groups are to the TPP, he doesn’t have enough leverage over Senate Democrats to force them to accept a deal they hate.