Lofven, who led Sweden from 2014-18 in a minority coalition with the Greens, agreed a deal last week with the Liberal and Centre parties to support him as prime minister. It included a clause explicitly excluding the Left Party from influencing policy.
"They want our votes in order to take power, not to give us any say. That is totally unacceptable," Left Party leader Jonas Sjostedt told reporters ahead of a PM vote on Wednesday.
However, he held the door open for a deal and said negotiations in the next few days would be decisive.
"Our goal is that Stefan Lofven becomes prime minister, but we are not yet at a point where we can vote for him," Sjostedt said.
In a sop to the Left Party, Lofven said Sjostedt had misunderstood and that the agreement still left room for cooperation in areas outside the budget and a 73-point programme agreed with the Centre, Liberals and Greens.
But he said the four-party agreement could not be renegotiated.
"The alternative is not Social Democratic politics, it is Moderate, Christian Democrat government that relies on the Sweden Democrats," Lofven said.
September's election delivered a hung parliament and the centre-left and centre-right blocs have been at loggerheads on how to form a government without the support of the anti-immigration Sweden Democrats, a party with roots in the white-supremacist fringe and which holds the balance of power.
The speaker has previously said he would nominate a new candidate for prime minister on Monday and that a vote would be held on Jan. 16. The speaker will hold a news conference at 1400 GMT.
If parliament rejects that candidate, the speaker can make one more nomination before a snap election must be held.
Polls show the Sweden Democrats could increase their support in a fresh vote, while Moderate leader and centre-right prime minister candidate Ulf Kristersson said on Monday that a snap election would undermine faith in the democratic process.
The election commission has suggested April 7 as a date an election could be held.
GRAPHIC - Election scenarios: https://tmsnrt.rs/2p45tJh
(Reporting by Johan Sennero, Johan Ahlander and Simon Johnson; editing by Niklas Pollard and Ed Osmond)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)