By Bertrand Benoit 

BERLIN--The verdict of Greek voters on Sunday presents German Chancellor Angela Merkel with her toughest challenge since the eurozone crisis broke out five years ago.

How she responds in the coming days won't just determine the fate of the eurozone, but will reflect on her as well, both at home and abroad, and it is hard to imagine a scenario where she emerges unscathed.

Given how much Germany has shaped the management of the crisis--a mixture of emergency loans and unpopular economic overhauls in the affected countries--the strong "no" vote emerging in Greece, according to a partial count of ballots Sunday, was a stinging blow.

A day earlier, the leading German newsmagazine Der Spiegel ran a cover story that lay the responsibility for the future of Greece and of the eurozone firmly at her feet. The magazine accused her of provoking the crisis through an approach it called "pedagogical imperialism."

Throughout the crisis, Ms. Merkel has repeated a mantra that "if the euro fails, Europe fails." In parliament last week, she said she would reopen talks with the Greek government after the referendum.

On Monday, she will travel to Paris to consult with French President François Hollande. She faces two options.

Yielding some ground on the terms of a new bailout, in particular by pledging some of the debt relief Greece and the International Monetary Fund have been asking for, could still save Greece from a devastating exit from the eurozone.

Yet while such a deal might secure the required approval of the German parliament thanks to opposition votes and those of Ms. Merkel's Social Democratic coalition allies, it would face considerable opposition in the chancellor's own conservative ranks. And it would still require a firm commitment to economic overhauls an emboldened Greek government is now unlikely to give.

Conversations with lawmakers in the past week suggest many conservatives might have rejected even the tough terms then under discussion. Any deal that requires lawmakers to pump more taxpayer money into Greece while seeing some of their past loans go up in smoke could spark a full-scale rebellion, lawmakers say.

"For a successful rescue operation, the one who wants to be rescued must let himself be rescued," Gunther Krichbaum, the conservative chairman of parliament's European Union Affairs Committee and advocate of a Greek exit from the eurozone, said last week. "As Greece obviously doesn't want this, there's no option left for those who want to rescue it."

When parliament convened last week for a debate on Greece, Ms. Merkel's cautious speech gathered tepid applause from the conservative benches--nothing like the thunderous clapping that greeted Wolfgang Schäuble, the chancellor's uncompromising finance minister.

Broader public support for a fresh Greek bailout isn't guaranteed either. While several polls published last week showed Germans were split on whether Greece should exit the euro, up to three quarters rejected further concessions to Athens. Mr. Schäuble, the embodiment of German intransigence in Greece, received his highest rating ever.

A sweetened bailout could be particularly damaging for Ms. Merkel because it would invalidate the very rationale for Germany's approach to the crisis: that it can only be fixed if uncompetitive economies are rebuilt and the eurozone's fiscal rules never bent again.

In case of a Greek exit, German voters are sure to put the blame largely on Greek Premier Alexis Tsipras, as recent polls indicate they have done so far.

Given all that, principles and an instinct for self-preservation may persuade Ms. Merkel to opt for the second option and stick with her tough line, an outcome many analysts see as more likely.

In a research note published on Sunday, Deutsche Bank said the most probable result of a "no" vote would be the end of Greece's euro membership, followed by the toppling of the Syriza government as economic hardship mounts.

In private, Berlin officials have also warned about this scenario in case of a no.

In an interview with the Bild tabloid on Saturday, Mr. Schäuble played down the risk of a Greek exit for the rest of the euro area and said remaining eurozone members would keep helping Greece. A separate interview by the Social Democratic Foreign Minister Frank-Walter Steinmeier warned of the "disastrous" signal such an outcome would send, highlighting a potential conflict in Ms. Merkel's "grand coalition" of the two biggest parties.

"Even with the best will in the world," the coming talks "will be a long and difficult process," one senior government official said.

Write to Bertrand Benoit at bertrand.benoit@wsj.com