Last night, liberal Democrats nearly blew up a proposed spending bill over its repeal of a relatively narrow provision in the Dodd-Frank financial-reform law — Senator Elizabeth Warren and House minority leader Nancy Pelosi led the opposition, but the bill ultimately passed and should pass the Senate before early next week, averting a government shutdown. Why did they pick a fight over this regulation?

I explained last night what the regulation actually is — it prevented specific parts of banks from dealing in a kind of “swap,” a financial instrument. And I suggested it was an appealing fight for someone like Warren, but not one of great policy significance: It’s hard to say if the costs of making it harder for banks to deal in this business are outweighed by the benefits of making sure they have to keep the business separate from their federally insured subsidiaries.

Risking a government shutdown over this little fight made it look like Warren may have been more interested in raising her own profile than pushing policies liberal financial reformers have been looking for. (By contrast, Ted Cruz’s attempt to defund Obamacare last year made much more sense for conservatives who thought it might work: It was about stopping Obamacare in its entirety.)

But that isn’t the only reading of Warren’s decision. Matt O’Brien of Wonkblog contends that there’s a regulatory principle at stake here — don’t let federally backed banks make exotic bets — which seems to understate how complicated, and yet practically not all that meaningful, these distinctions are. But he makes a good point that Warren may be trying to establish a political principle:

The bigger issue is whether every legislative choke point will turn into another opportunity to whittle away at financial reform. That’s why Warren and her allies have decided to draw a line in the sand here, over swaps push-outs. If they don’t fight this battle, then they’ll have to get ready for the next one over even more important parts of Dodd-Frank—and so on, and so on.

It seems like Wall Street could have been thinking something similar when J.P. Morgan CEO Jamie Dimon started making phone calls to lobby against Warren and Pelosi last night. John Carney of the Wall Street Journal suggests the fight the banks put up may not have been worth it, such was the political capital expended. It did briefly put bank deregulation in the light of day, and that remains controversial six years after the financial crisis, as, to some extent, it should. Whatever Warren’s and Wall Street’s reasoning, though, we probably haven’t seen the last of this kind of fight: They both have a lot at stake.