The divisive issue came to the floor on the day lawmakers also vote on belt-tightening measures needed for the quick release of a USD five-billion IMF payment that could spare cash-strapped Kiev from slipping into default.
The sudden flurry of activity led visiting US Assistant Secretary of State Victoria Nuland to call today "yet another historic day" in a war-torn country that has seen 6,500 people killed since the overthrow of its Russian-backed leadership in February 2014.
The idea of granting autonomous status to rebel-run parts of Ukraine's east for the coming three years has struck a note of disquiet among many lawmakers and much of the Kiev media.
But it was inscribed in a truce agreement that Poroshenko and Russian President Vladimir Putin signed off on under strong pressure from the leaders of Germany and France in February.
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Lawmakers today voted by a commanding 288-57 majority to ask Ukraine's constitutional court to rule whether such changes to the basic law were legal.
Both Washington and its EU allies believe that partial self-rule could satisfy the insurgents and remove any arguments Russia may have for arming and funding them -- support Moscow firmly denies ever giving.
But Leonid Yemets of parliament's pro-government People's Front party said he and other top lawmakers had told Nuland her logic was flawed because it failed to take ongoing hostilities into account.
"First we must end the war... And only then conduct (local separatist) elections and make corresponding changes in the constitution," the lawmaker said.
"We cannot create a situation in which we end up facing the aggressor (Russia) on our own," the Ukrainian leader said.