From The Money Game:

Wharton is warning markets to keep a very close on Spain right now. That's because while a Greece or Portugal financial melt down might be manageable, a Spanish one could have massive negative repercussions for both Europe and the global economy.

This is due to the massive size of both Spain's economy and debt:

If Spain fails to execute a credible plan to cut its budget deficit, the worries over sovereign solvency will spread quickly beyond the small, peripheral countries currently making the most headlines, experts warn. A Spanish default could herald the breakup of the euro and...

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More on the euro crisis:

Jim Rogers: Let Greece go bankrupt...

This could be the next country to fall apart

Doug Casey: European Union is going to implode soon