Lawmakers from Keiko Fujimori's Popular Force party released recordings of her brother Kenji and others apparently negotiating public works contracts with a government official, and said it was proof that Kuczynski was trying to buy votes to defeat impeachment.
The images "show the government buys MPs to stay in power," said Moises Mamani of the Popular Force party.
"This government thought it could buy everything. There will always be brave and worthy Peruvians who are not for sale. It's time to tell Mr PPK that he's leaving, and NOW!" Keiko Fujimori tweeted, using the president's initials.
The videos, dating from December, were filmed ahead of a previous impeachment vote that Kuczynski survived, with Kenji Fujimori's help.
Days later, Kuczynski pardoned the Fujimoris' imprisoned father, ex-president Alberto Fujimori, in what was widely seen as a payoff for support.
The government denied vote-buying. But analysts said the damage had been done, and the president could even announce his resignation ahead of the vote.
"Legislators that were undecided about this new impeachment attempt are abandoning ship," Eurasia Group's Maria Luisa Puig said.
"The opposition will probably oust him in an impeachment vote tomorrow, though his resignation before then is possible."
Kenji and a group of lawmakers were thrown out of the party by his sister, and he has now formed his own party. Keiko narrowly lost out to Kuczynski in the 2016 presidential election.
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