Few Americans will have heard of John McDonnell MP. Yet a demonstration he attended in support of Guantanamo Bay detainees outside the U.S. Embassy in London last week shows why they should begin to take notice. This far-left firebrand is now just a Conservative government implosion away from becoming the second most powerful politician in the UK, and that spells bad news for U.S.-UK relations.

Until recently, McDonnell was just another leftist revolutionary operating on the fringes of a mainstream party (in this case, the UK’s Labour party). However, following a landslide victory for fellow hard leftist Jeremy Corbyn in the party’s leadership elections last year, McDonnell is front and center in the UK political scene. Now the Shadow Chancellor in Her Majesty’s Opposition is a man who boasts of “fermenting the overthrow of capitalism” and fondly quotes Mao Zedong on economic policy.

McDonnell’s stance on security and terrorism is of a piece with his distaste for capitalism. In 2003, he said “it was the bombs and bullets and sacrifice made by the likes of Bobby Sands that brought Britain to the negotiating table,” a reference to the IRA hunger striker who died in jail. Now, McDonnell is lending his support to a new set of terror suspects: British citizens and residents formerly detained at Guantanamo Bay.

The anti-Guantanamo demonstration McDonnell attended last week was organized by the Islamist pressure group CAGE.  The group’s “research director” has characterized ISIS butcher Mohammed Emwazi (aka “Jihadi John”) as ”gentle,” ”kind” and ”beautiful.”

Another staff member at CAGE is Moazzam Begg, himself a former Guantanamo detainee.

Indeed during the protest, the Shadow Chancellor was photographed alongside five former Guantanamo Bay detainees. Let’s take a look at his Gitmo-graduate associates, starting with Begg.

Currently CAGE’s director of outreach, Begg was detained in Afghanistan, where he signed an FBI confession admitting to fighting and training alongside al-Qaeda. (He now repudiates the statement, which he says came about due to torture. Multiple US government investigations reject this claim.) Begg has previously spoken of his time visiting camps in Afghanistan and Pakistan and training rebels in Syria. Recently he has been pushing the idea that the best way to discredit ISIS is to use, in his words, ”Islamic clerics traditionally associated with al-Qaeda”: the notorious jihadist scholars Abu Muhammad al-Maqdisi and Abu Qatada.

Then there is Shaker Aamer, a suspected al-Qaeda member originally from Saudi Arabia. According to the Department of Defense, he fought in Bosnia, hooked up with the mujahideen in Afghanistan and then fought against the U.S. and its allies in the caves of Tora Bora post-9/11. The Pentagon believes that Aamer fought under the command of the Libyan terrorist Ibn al-Sheikh al-Libi. (Aamer denies all these allegations.) According to former Guantanamo Bay warden Mike Bumgarner, Aamer’s gravitas at the camp was such that just the Saudi’s presence would move hardened jihadis to tears. This video from the CAGE demonstration proves that this ability to make grown men cry endures.

The other three men with McDonnell are known as the “Tipton Three” — a reference to the small West Midlands town where they lived. Of these three, Ruhel Ahmed and Shafiq Rasul admit to firing guns at a Taliban training camp; while Asif Iqbal told U.S. interrogators that one of his hobbies was ‘killing Americans’ and that he had left the UK after 9/11 to “fight with bin Laden.”

McDonnell was not hoodwinked into turning up at the embassy protest. Indeed, he was already co-chair of a ‘Save Shaker Aamer’ Parliamentary Group. During an April 2013 parliamentary debate,  he suggested “that the Foreign Secretary should be summoning the American ambassador now, to say that this Government have had enough…If that causes an international incident, I do not care anymore.”

Conventional wisdom in the UK says that all this, as outrageous as it is, should not matter much. The Conservative party won the general election last year, while Corbyn and McDonnell are so radical that they are virtually unelectable.

Perhaps. Yet Tory complacency will inevitably lead to unforced errors. The party is already poised to revive one of its regular internecine wars over Europe, with a referendum over EU membership likely to lead to sharp divisions within the party. Furthermore, as Prime Minister David Cameron has pledged not to see out a full term in office, there is also a leadership battle on the horizon.

It is now genuinely a matter of national security that the impact of such ruptures be minimized. As McDonnell proved again last week, the stakes are too high for Labour to be presented with a shot at power.

- Robin Simcox is a research fellow in The Thatcher Center for Freedom.

- This piece originally appeared on The Daily Caller.