BERLIN — Chancellor Angela Merkel, a Protestant pastor’s daughter, grew up in East Germany under a system in which President Vladimir V. Putin of Russia served as a K.G.B. agent stationed in Dresden. Fluent in Russian, she literally speaks his language and, arguably, understands his view of the world better than any other European leader touched by the Ukrainian crisis.

With that shared background, not to speak of the extensive economic ties between the countries, Ms. Merkel has emerged as a pivotal leader in the crisis that erupted after Russian forces overran Crimea late last month, the only one whose advice Mr. Putin might heed. But for all the familiarity, and despite hundreds of hours spent together during their years as leaders of their respective countries, Ms. Merkel has so far been unable to bridge the gap with Mr. Putin, whose German is also good.

If anything, as the days dwindle before a Crimean referendum on secession, that gap is widening, and now Ms. Merkel is facing a perhaps historic decision on whether to take a harder line against Russia.

Since the first Russian forces infiltrated Crimea on Feb. 28, Ms. Merkel, 59, has spoken to Mr. Putin, 61, at least four times on the phone, her spokesman says. In the space of 10 days, she went from warning him to avoid “any step that could contribute to escalation” to bluntly telling him that Crimea’s plans for a referendum on joining Russia are “illegal.”