MADRID—Greece set off the crisis rattling the euro zone. Spain could determine whether the 16-nation currency stands or falls.

The euro zone's No. 4 economy, Spain has an unemployment rate of 19%, a deflating housing bubble, big debts and a gaping budget deficit. Its gross domestic product contracted 3.6% in 2009 and is expected to shrink again this year, leaving Spain in its deepest and longest recession in a half-century.

At...