Nearly all MPs from the Republican Party of Armenia voted against appointing the leader of the Velvet Revolution as the head of the government. In a concession last week, the Republicans agreed to support any prime minister candidate nominated by a third of the parliament members, paving the way for Pashinian's election.
Prime Minister Nikol Pashinian faces an array of challenges, including a parliament dominated by the party he denounced as corrupt and how to resolve the volatile question of Nagorno-Karabakh, a region of Azerbaijan under the control of ethnic Armenian forces.
Moscow, which has a military base in Armenia, is wary of an uncontrolled change of power which would pull the country out of its orbit, but Pashinyan has offered assurances that he will not break with the Kremlin.
Under Armenia's constitution, if a prime minister is not elected in the second vote, parliament will be dissolved and early general elections would be held with the HHK-led acting government in charge of the electoral process.
The head of the ruling Republican Party's parliamentary faction, Vagram Bagdasaryan, said his party backed Pashinyan to "ensure stability" in the country. The previous Prime Minister Serzh Sargsyan resigned amid mass demonstrations of opposition supporters.
Analyst Vigen Akopyan said snap elections looked certain.
Nikol Pashinyan was a famous journalist in Armenia Editor-in-Chief of opposition newspaper "Haykakan Jamanak" (Armenian Times).
The protests which brought the resignation of Serzh Sargsyan were the biggest in Armenia since the Declaration of Independence in 1991 and started as sudden movement, unxpected by both experts and the political opposition. Since 13 April, tens of thousands have come out to the streets daily in what Pashinyan has called a "velvet revolution".
"There will be no privileged people in Armenia".
Pashinian went into hiding following the deadly suppression on March 1-2, 2008 of post-election protests in Yerevan. "The victory is that you chose who would be prime minister".
"This is a real test case for Russia's politics in the former Soviet Republics", said Chatham House's Broers.
The Republican Party, which has a majority of parliament seats, later underlined the difficulty by declaring it's now in opposition.