To listen to harried European Union chief Donald Tusk is to realize that the leaders of the 28 member nations are facing the prospect of an unhinged EU if this week's summit goes badly wrong.

Negotiations with Britain over EU reforms to give Prime Minister David Cameron a better shot at winning a referendum to stay an EU member are far from finished as they head into the final stretch.

At the same time, the migrant crisis is quickly turning into a seminal test of unity, pitting east against west and many against Greece.

"The migratory crisis we are currently witnessing is testing our union to its limits," Tusk said Tuesday as he was seeking compromise on both issues ahead of Thursday's summit.

"There is an extra mile we will have to walk to reach an agreement," Tusk said in Prague Tuesday, near the end of a five-nation whirlwind tour that was to end in Berlin barely 24 hours after it started in Paris.

Instead of the veneer of unity, the EU can no longer hide its divisions.

A British referendum could come as early as this summer, and the loss of one of the biggest member states -- a member of the G7 group of richest economies and the United Nations Security Council -- could be sealed.

It would be one massive blow. When it comes to migration, it is more a steady crumbling of foundations that is feared.

Four eastern European nations have publicly taken stand against some of the core EU policies and publicly chided southern member Greece. Hungarian Prime Minister Viktor Orban recently said that "if it were up only to us Central Europeans, that region would have been closed off long ago," referring to the south where the migrants come from.

Hundreds of thousands of migrants have come, almost unchecked, through EU member Greece and on to Germany and Sweden. Almost everyone complains Greece isn't doing enough to stem the influx. The rich member states who are the draw for the migrants complain the eastern Europeans are not doing their part to shelter refugees. Several eastern European nations complain that they lack the resources to handle large numbers of refugees and that the more prosperous nations are too soft-hearted and have allowed the borders to be overrun.

And just about every country has said that the European Union has failed miserably to deal with the migrant crisis.

The us-versus-them thinking has increasingly become a mind-frame within the EU. Sharp divisions have become commonplace in part carried over from the financial crisis, which some cast as a clash between profligate states like Greece against penny-pinching Germany.

Tusk came to the defense of Greece on Tuesday and told the Visigrad east European group of nations -- "Let me be very clear: Only when united can we solve this crisis."

And if Tusk was already instrumental in brokering a deal for Greece last summer, he feels the current crises are an even bigger test.

The stakes are immense, Tusk acknowledges, fearful that if Britain goes it will start an unraveling that no one knows when and where it might end. A so-called Brexit might turn into a full-blown EUxit.

The European Union was built on the ashes of World War II, first taking decades to bring economic wealth before taking on the task of bridging the huge ideological divide that cut the continent into a capitalist west and a communist east.

As a Pole, Tusk himself was reared under communism before the collapse of the Soviet empire. Barely a dozen years ago, Poland joined the EU at the very height of the bloc's powers. Now, he does not want to oversee the collapse of what he so dearly believes in.

Everyone has always known the British to be argumentative and not easily given to compromise. "I make no apology for that. That is who we are," Cameron said last week.

The problem is that this stubbornness is now seeping into the cornerstones of the EU ever more.