Over the weekend, President Donald Trump launched into another Twitter rant, this time attacking the legitimacy of a “so-called” federal court judge who on Friday issued a nationwide injunction blocking the Refugee and Visa Order cooked up by Trump and his wingman Steve Bannon.

Federal District Court Judge James Robart is a 2003 George W. Bush appointee. Like Sally Yates—the former acting attorney general who also disagreed with the constitutionality of the order, known as the Muslim ban, and who was then promptly fired by Trump—Robart is a civil servant widely respected by Republicans and Democrats alike. In fact, the likely future Attorney General Sen. Jefferson Beauregard Sessions III (R-AL) voted for his confirmation.

And while it would be easy to write off Trump’s latest Twitter tirade as just that—Trump’s latest Twitter tirade—it would be exceedingly foolish to do so. After all, each one aims to stifle opposition and dissent. So when those temper tantrums are directed at the federal courts, and specifically federal judges who issue orders in opposition to Trump’s policies, we must pay attention to them.

Trump’s latest outburst matters for several reasons. First, it is part of an ongoing effort by the administration to de-legitimize the institution of the federal courts. This is important, because we’ll likely see a lot of Trump’s agenda end up in federal litigation.

Look at the challenges to the Muslim ban alone. In addition to the numerous other petitions filed on behalf of individuals immediately affected by the ban, Trump’s executive order is already in front of a federal appeals court. Almost immediately after Robart issued the injunction, the Trump Department of Justice asked for a stay of that order from the Ninth Circuit Court of Appeals, the federal appellate court that hears cases from, among others, Robart’s district. The Ninth Circuit refused that request immediately and entirely. It follows, then, that while we are many steps from getting there, Trump’s Muslim ban is likely to end up before the U.S. Supreme Court at some point. Given the fact we’re not even a full month into Trump’s presidency, and the additional campaign promises he clearly intends to keep—like signing unconstitutional abortion bans—there will almost certainly be more of Trump’s policies before the Court too.

This is important because President Trump has sent a very clear message to federal judges: Try and block my policies and find yourself on the receiving end of executive-branch wrath. Trump didn’t send one stray tweet, after all: He launched into a series of tweets suggesting that should another foreign terrorist attack happen on U.S. soil, it would be the fault of the courts for disobeying Trump and blocking the executive action. This is unchartered waters for our democracy; it has the potential to undermine the necessary check the federal courts serve on the executive branch. Quite simply, Trump is trying to bully federal judges out of ruling against him and suggesting those who do are not “legitimate” members of the federal courts.

Second, Trump’s attacks on the federal courts matter because those attacks have the power to influence the culture and composition of the courts themselves. Last week, Trump nominated federal appeals court judge Neil Gorsuch to fill the seat left vacant by Antonin Scalia’s death. It’s not enough for Democratic members of the Senate Judiciary Committee to ask Gorsuch during his hearing if he disagrees with Trump’s comments; Senate Democrats need to ask him point-blank if he’s independent from the president, or if he was nominated because Trump and Bannon feel confident he will rule as they need him to.

And this approach needs to extend to all of Trump’s other court nominees. By suggesting that the federal courts are illegitimate because they disagree with his policy, Trump makes suspect every single one of his nominees moving forward. Why would Trump nominate a judge he thought for a second would not move his agenda along, Constitution be damned?

Then there is the possibility of impeachment. After all, every day Trump remains in office he’s violating the Constitution because he hasn’t fully divested of his private business dealings, including dealings in places specifically left off Trump’s Muslim ban.

So let’s say it happens. Let’s say somehow, Republicans in the House find some courage and actually do draft Articles of Impeachment. That impeachment trial will happen in the Senate, where Chief Justice John Roberts will preside over the proceedings.

And let’s say those proceedings end up with Trump impeached. Will Trump, Bannon, and his supporters accept that ruling and leave office? Or will this “so-called federal judge,” aka Roberts, be attacked, and the order removing Trump from office ignored?

Like Trump’s attacks on the media and the “fake news” narrative, the attacks on the federal courts are designed to both chill opposition to Trump’s policies and to call into question the institutions set up to check the power of the presidency. Trump can’t fire Judge Robart the way he did Sally Yates, so instead he’s attacking Robart’s legitimacy. We’ve seen this, too, with Trump’s approach to the New York Times, CNN, and any political reporting or polling that shows disagreement with his agenda.

That’s also why the press and the courts are more critical than ever. Because Trump can’t fire them, they are the two institutions which have the greatest ability to act as the firewall to Trump and Bannon completely laying waste to our democratic ideals.

CORRECTION: A previous version of this article mistakenly referred to the former acting attorney general—Sally Yates—as Sarah Yates. We regret the error.