Party officials said over the weekend that Republican lawmakers would vote as a bloc, leaving uncertain prospects for Pashinian.
On Monday, thousands of Armenians marched through the capital, demanding that Mr Pashinyan be appointed prime minister.
Opposition leader and founder and head of Civil Contract party Nikol Pashinyan leads a street campaign against the ruling authorities, which has seen thousands of Armenians taking to the streets in massive protests. On 23 April Serzh Sargsyan, the then-prime minister of Armenia, resigned.
Pashianian insists that only an interim government led by a "people's candidate" - himself - can organize and conduct parliamentary elections that are free, fair, and transparent.
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Speaking to lawmakers ahead of the vote, Pashinian said that "the Republican Party's attempts at thwarting this ballot will start a political tsunami".
However, as a debate unfolded in parliament to discuss Pashinyan's candidacy for the vacant prime minister's post, it became clear the ruling elite was not willing to surrender power. However, he needs 53 votes from the 105-seat parliament to rise to premiership.
"I was hoping to hear a conceptual speech of a candidate for the prime minister, but instead I was witness to a non-program speech which sounds pleasant to people's ears", senior Republican party member Eduard Sharmazanov told the chamber.
Ex-Soviet Armenia has been in the grip of a severe political crisis for the past few weeks, with leader Sarkisian stepping down last week after a decade in power in the face of peaceful protests.
They also accuse Sarkisian and his dominant Republican Party of widespread corruption and failing to raise living standards. Parliament was due to have another election within a week. At the meeting, Sargsyan presented the reasons of his recent resignation in detail and spoke about different formats of working in the parliament - urging to prioritize domestic stability and security of the country in everything, which in case of any decision can be implemented only through unity of Republicans. "It would be the final stage of the people's victory", he said in parliament.