The 54-year-old, who took over as leader of the Social Democrats earlier this month, reached a deal with the Left Party late on Tuesday to raise pensions in exchange for its backing in Wednesday's vote in parliament.
"We have reached an agreement to strengthen the finances of the poorest pensioners," Andersson told public broadcaster SVT minutes after the deal was announced.
"We're not going to block Andersson," Left Party leader Nooshi Dadgostar told Swedish Radio.
Under Sweden's system, a prime ministerial candidate does not need the support of a majority in parliament -- rather, they just need to not have a majority against them.
Andersson has already received the support of the Social Democrats' coalition partner the Greens, as well as the Centre Party.
However, political observers noted there was still a slight chance the Centre Party could thwart Andersson's bid.
It has previously warned that it could withdraw its support if she concedes too much ground to the Left Party.
Centre Party leader Annie Loof refused to comment late Monday on the Left's deal with Andersson.
The vote will take place at 0800 GMT.
If elected, Andersson would formally take over her functions following a meeting with King Carl XVI Gustaf on Friday.
She would replace Stefan Lofven, who resigned on November 10 after seven years as prime minister in a widely expected move aimed at giving his successor time to prepare for the country's September 2022 general election.
More about: Sweden
