Brian Klaas is a fellow in comparative politics at the London School of Economics and author of the forthcoming book "The Despot's Apprentice: Donald Trump's Attack on Democracy." The opinions in this article belong to the author.

(CNN) The President of the United States woke up this morning and decided to retweet three videos from the account of Jayda Fransen -- the deputy leader of Britain First: a far-right, ultranationalist, neo-fascist hate group.

Britain First organizes "Christian patrols" and "invades" mosques trying to intimidate those who attend religious services.

Trump gave her a platform -- spreading her hateful message to his 43 million followers.

The videos he retweeted paint Muslims as anti-Christian criminals who beat up white kids and break statues of the Virgin Mary. They are clearly aimed at demonizing Muslims, a religious minority that already faces a disproportionate number of hate crimes.

Let's be crystal clear about this: Trump's behavior is sick, disgusting and disqualifying. We are watching, in real time, the President of the United States using his power and platform to mainstream the vile ideologies of racist neo-fascism.

This isn't coming from the political fringe any more; it's coming from the political center of the United States: the Oval Office.

Have you no decency, Mr. President?

The answer, of course, is no -- he does not. But if we continue to accept these reckless, divisive outbursts as part of our normal political discourse, then we will have answered the question as to whether we have any decency left too. We will have failed yet another test.

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Now, just as the neo-Nazi website The Daily Stormer celebrated after Trump made his now infamous "many sides" comments after Charlottesville -- in which he said there were "very fine people" at a neo-Nazi rally that murdered a peaceful protester -- Trump's retweets have Britain First's racist following celebrating too.

In a year where we seem immune to being shocked by the sheer depravity of Trump's actions, this latest action must shock us.

First off, it's un-American. The First Amendment guarantees religious freedom. After all, the United States was founded by Pilgrims fleeing religious persecution.

Secondly, it's dangerous. During the 2016 campaign, as Trump attacked Muslims and called to ban them from entering the United States, hate crimes against Muslims rose by nearly 20% . That disturbing trend is more likely to continue after the President used his public office to suggest that Muslims are vicious anti-Christian criminal deviants.

Trump's actions also damage American security. When Trump vilifies Muslims, it damages our relationships with Muslim-majority nations -- who already are uneasy about Trump's previous rhetoric.

But it also does serious damage to America's power in the world by alienating allies and forcing them to turn away from Washington. Here in the UK, Britain First is correctly viewed by most as an extremist, racist hate group with no place in British politics. Shortly after Trump's tweets, at least one member of Parliament condemned Trump and said he was not welcome to visit.

The UK has long been America's closest ally, with Brits and Americans fighting and dying together to defeat fascism. Now, Britain is being forced to turn away from the United States in horror as the President promotes neo-fascist hate.

This isn't a drill. Nor is it not some curiosity to shrug at, or look on in disgust as we get on with our days. When are we going to stop this madness?

Congress must grow a spine. Checks and balances only work if people are courageous enough to stand up to a demagogue. A "Decency Caucus" -- those elected officials in both parties who believe in core American values and democratic norms -- should be formed to send a clear message to Trump. No legislation should be passed until he apologizes for promoting the hate-filled bile of a neo-fascist group and takes the tweets down.

Today is yet another low point in a presidency defined by its constant low points. Republicans and Democrats can agree to disagree on taxes, health-care policy or foreign affairs. But we must stand united in opposition to Trump's bigotry, his persecution of religious minorities and his constant attacks on the core values of American democracy.