MADRID  Europe so far has survived the bailout of Greece. The financial rescue of Ireland also is manageable. Even if Portugal becomes the third country to succumb and seek aid, as many people widely predict, it is unlikely to push Europe to the financial brink.

But any bailout of Spain  with an economy twice the size of the other three combined  could severely stress the ability of Europe’s stronger countries to help the financially weaker ones, and spell deep trouble for the euro, Europe’s common currency. Even though Spain, like Ireland, has adopted an austerity plan to help it avoid the need for a bailout, it still could need aid if its banking system proves frailer than the government thinks it is, as was the case in Ireland.

This troubling possibility has unnerved lenders, with Spain’s borrowing costs rising even though Madrid has cut its deficit and the country’s banks maintain they have sufficient strength to absorb their bad real estate loans. “Europe can afford the collapse of Ireland, even perhaps that of Portugal, but not that of Spain, so Spain’s ultimate line of defense is in fact this knowledge that it’s too big to fail and that it represents a systemic risk for the euro,” said Pablo Vázquez, an economist at the Fundación de Estudios de Economía Aplicada, a research institute here.

Reflecting the worries of investors, the yield spread between Spanish 10-year government bonds and those of Germany continued to widen on Wednesday  to as high as 2.59 percentage points, the biggest gap since the introduction of the euro. Spreads typically widen when investors perceive greater risk of not being repaid.