Trump is right to pick a fight with Rahm Emanuel.

A basic constitutional primer: Article 6 of the Constitution provides that where federal and state law conflict, federal law is supreme.

That makes federal law the supreme authority of the land. Correspondingly, though it may take a Supreme Court ruling to eventually decide the issue, President Trump will be vindicated in denying federal grants to sanctuary cities. After all, by granting illegal immigrants safe haven against lawful federal deportation rulings, those cities are obstructing the law.

Mayor Rahm Emanuel of Chicago disagrees. On Monday, announcing a lawsuit to prevent the Trump administration's withholding of a multimillion dollar grant, Emanuel pledged never to be "coerced" into suspending Chicago's sanctuary city status.

Like so many other sanctuary city leaders, Emanuel's position is ridiculous and immoral in equal measure.

For a start, the Trump administration is not forcing Emanuel to do anything. Instead, it is qualifying the provision of federal grants to the city's conformity with the law. By actively obstructing the activities of federal law enforcement, Chicago is rejecting a basic principle of the law. Removing federal grants is thus a small but necessary step to encourage sanctuary cities to comply with the law. If Emanuel wants to continue his sanctuary city status, he should not expect to be rewarded for his rejection of the law.

Of course, there's a broader issue at stake here. When it comes to illegal immigration and sanctuary cities, left-wing politicians have been allowed to have their cake (federal grants) and eat it (accept illegal immigrants) for far too long. They talk a great deal about human rights, but happily ignore the fact that functioning society rests on the supremacy of just law. Trump has a compelling state interest in confronting those who take the opposite approach.

Ultimately, Trump doesn't just have the right to challenge Emanuel and every other sanctuary city mayor, he has the responsibility to do so. While Rahm might care only about illegal immigrants, the Americans who elected Trump expect that their tax dollars will not be used for left wing pet projects. On both its moral and legal merits, this is a political battle that Trump will win.