As Elizabeth Warren started to rise in the (still very early) polls, a number of Bernie Sanders-supporting pundits closely parsed the two candidates’ policies and rhetoric hoping to build a case that the Massachusetts senator wasn’t a real progressive. It’s understandable given that two movement progressives advancing deep into the primaries would likely split the left’s vote and allow a more moderate candidate to grab the nomination.

The resulting arguments weren’t terribly persuasive; many focused on labels—Warren identifies as a progressive and as a capitalist while Sanders is a European-style social democrat. Some criticized Warren for co-sponsoring Sanders’ Medicare-for-All bill but not talking about it often enough on the campaign trail.

But the central problem with drawing these fine-grained distinctions is that Democrats aren’t nominating someone to run for a dictator. If the party pulls off the difficult task of holding the House and winning control of both the Senate and the White House next year, it will advance whatever policies can get through a Congress that will have dozens of Blue Dogs and New Dems in the Dem caucus. They won’t have 218 movement progressive votes in the House, nor a progressive super-majority in the Senate. Nobody is really arguing that Warren would be less likely than Sanders to put her weight behind the most progressive bills considered by Congress, or that she would be less likely to sign those that passed into law. Given the same Congressional constraints, there’s no obvious reason to believe that the outcomes would be significantly different if Sanders or Warren were sitting in the Oval Office. That isn’t the case with the other leading candidates.

Which brings us to a significant difference between the two: Their respective theories of overcoming both Republican obstruction and resistance from moderates within their own party. While Sanders’ embrace of social democracy makes him somewhat of a radical by American standards, he has so far proven to be institutionally conservative relative to Warren.

Warren was the first candidate to explicitly call for killing the filibuster. In February, Sanders was asked about the idea in an interview with CBS. “No, I’m not crazy about getting rid of the filibuster,” he said. Since then, he has warned that people should be “a little bit nervous” about ending the filibuster given that Trump supports doing so. “Whether you’re in the majority or the minority, I think you have to protect minority rights. I don’t think you can just simply shove everything through,” he said, adding: “There’s an argument for that, by the way, but that’s not where I am right now.” (Sanders says that policies like Medicare for All and The Green New Deal can be passed through budget reconciliation, which requires a simple majority in the Senate. But this is a dubious proposition.)

Anyone who lived through 2009 and 2010 should recall that a host of good, progressive bills were passed in the Democratic-controlled House only to be watered down or killed off in the Senate. That happened despite the fact that Dems had a filibuster-proof majority for some of that period. Even if you have 60 votes, when you need every one of them to pass legislation it gives an enormous amount of leverage to the most conservative members of your caucus.

Earlier this year, The Washington Post surveyed the candidates’ positions on a number of measures to overcome Republicans’ structural advantages, as well as those gained by running roughshod over longstanding democratic norms. Warren is in favor of eliminating the Electoral College; Sanders is equivocal. Warren is “open to” the idea of adding Supreme Court justices to rebalance the Court after the GOP’s brazen theft of Merrick Garland’s seat; Sanders said he’s opposed to the idea. Warren was also open to imposing term limits on justices—one of the most moderate proposals for reform–Sanders came out for “modified term limits” in April.

AlterNet doesn’t endorse candidates, but in the interests of disclosure, I support Warren after backing Sanders four years ago. And while this pattern isn’t the only reason for that decision, it is the primary one. I believe that whether we’re talking about extreme gerrymandering or voter suppression or the filibuster or Wyoming (population 550,000) enjoying the same number of Senators as California (with almost 40 million people) or a president who lost the popular vote by a margin of three million ballots appointing 30 percent of the judges in the federal judiciary, if we don’t fight hard for a level playing field—for small-d democracy–we are not going to see much progress. Protecting minority rights in the Senate is a lofty goal, but you can’t bring a knife to a gunfight. And with the existential threat of catastrophic climate change looming over our heads, maintaining the institutional status quo is a luxury we can’t afford.

Sanders’ wariness about shaking up the structures of government is now doubt born of his consistent belief that pure economic left-populism, in the hands of the right politician, can attract a massive, indomitable political movement that will sweep away all opposition. It’s a romantic view of politics, but one that is unsupported by the data and that I find totally unpersuasive—and not only because it wasn’t enough to secure a Democratic primary against a polarizing opponent. (If you think the DNC robbed him, just keep in mind that they didn’t control the top-rated cable news channel in the country or a proliferation of online outlets like Breitbart and The Daily Caller.)

It’s unrealistic because most people don’t follow policy in great detail and fall back on partisanship to guide their views. Many, like the non-college educated whites who bristle at “coastal elites” but supported a sleazy billionaire real estate developer from New York, vote according to social grievances and cultural resentments of various flavors. Politicians tend to worry more about their own constituents than the national polls. There are numerous veto points built into our system, and there’s no reason to believe that Sanders could dissuade his opponents from using them. When it has happened–with women’s suffrage or the civil rights movement–it took place on a timeline much longer than a president’s four or eight years in office.

Warren, who fought tooth and nail for the creation of the Consumer Financial protection Bureau, seems to understand that you can’t lead a revolution without killing the filibuster. It’s probably the biggest difference between the two progressives in the race.