Spain’s parliament is due to begin debating whether to allow the Socialist party to form a new government.
The Socialists (PSOE) came second in December’s inconclusive elections and have until the end of the week to see if they can secure a majority in parliament.
On Tuesday, PSOE leader Pedro Sanchez appealed to deputies to back his bid to form a coalition.
However, correspondents say his chances of forming a government are slim.
If he fails in a confidence vote later on Wednesday, another vote will be held on Friday.
If neither vote is successful, parliament will have two months to choose a government or face fresh elections on 26 June.
In his address to parliament on Tuesday, Mr Sanchez called for the formation of a coalition based on common interests.
He said a Socialist-led government would enact a series of progressive measures such as a minimum wage increase and a gender wage-gap law.
“Why don’t we form a government based on those things that bring the majority of the 350 deputies of this parliament together?” he asked.
“Why don’t we unite to form a government for change? Why don’t we get together and pass as many reforms as possible to solve the most urgent problems of Spaniards? Why don’t we vote together in favour of all issues in which we are in agreement?”
Between them, the PSOE and partner party Ciudadanos command only 130 seats in the 350-seat lower chamber of parliament.
The two other main parties have said they will not support Mr Sanchez.
To become prime minister, Mr Sanchez will need the support of more than half of the chamber’s 350 deputies in Wednesday’s confidence vote.
Failing that, another vote will be held on Friday in which he would require only more votes in favour than against.
Given that opponents Podemos and the Popular Party have 191 seats between them, he would need at the very least significant numbers of their deputies to abstain.
But Podemos and smaller parties have so far rejected overtures from Mr Sanchez.
The incumbent Popular Party gained most votes in the 20 December election but its leader, Mariano Rajoy, was unable to secure enough backing to form a government.
The PSOE performed badly, hit by the emergence of Podemos and Ciudadanos, and the fragmented political landscape has eluded efforts to agree a governing coalition.