Lofven is trying to form a cabinet following an inconclusive election in September. He is now serving in a caretaker capacity after losing a mandatory vote of confidence that followed the election, but is trying to win support from traditional rivals the Centre Party and the Liberals to avert another election.
Centre Party leader Annie Loof said the Social Democrats had not yet satisfied her party's demands for reforms, which include lower taxes and less restrictive labour and housing laws.
"The Social Democrats' response to the Centre Party's political demands are unfortunately far from enough," she wrote in the daily Aftonbladet. "Taking that into consideration, we are ready to vote no ... but we will give the Social Democrats one final chance."
Last week Loof ruled out joining a Social Democrat-led government but said her party could abstain in the vote over Lofven's premiership if its demands were met.
Lofven's Social Democrats have just 100 seats in the 349-member Riksdag, with the remaining seats divided among seven parties, making forming a coalition difficult.
The Centre Party controls 31 seats, which could be enough to sink Lofven if it votes against him.
GRAPHIC - Election scenarios: https://tmsnrt.rs/2p45tJh (Reporting by Johan Ahlander; editing by Niklas Pollard)
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