Today, the House is set to vote on a resolution, drafted by Democrats, to overturn President Trump’s declaration of a national emergency at the southern border, which Trump put in place after Congress declined to provide $5.7 billion for the construction of hundreds of miles of border wall. The House vote will force the Senate to weigh in on the measure.

Trump has framed the dispute as an argument about border security. He wants a wall, but Congress won’t fund it. Yet in many ways, the border wall itself is a secondary issue. This is a constitutional conflict between co-equal branches of government over the nature of the relationship between the executive and the legislature.

To understand what’s at stake in these votes, it’s worth going back several years to a dispute between House Republicans and the Obama administration over the funding of Obamacare.