If Peru's Congress sets the impeachment process in motion it would make Kuczynski the highest-profile political figure to be punished in the Odebrecht scandal.
If Peru's Congress sets the impeachment process in motion it would make Kuczynski the highest-profile political figure to be punished in the expanding scandal surrounding Odebrecht, a Brazilian engineering and construction firm that admitted to paying millions of dollars in bribes in several Latin American countries to secure public works contracts.
"I will defend my moral capacity," Kuczynski, 79, said of Thursday's congressional session.
"The constitution, and democracy are under attack. We are facing a coup disguised as a couple of supposedly legitimate interpretations of the law," he said, joined by his vice presidents Martin Vizcarra and Mercedes Araoz.
Earlier he lodged a judicial appeal to try to prevent Congress from moving ahead, arguing that "I have been convicted in advance," according to Peruvian media publishing a copy of his challenge. The outcome of the appeal was not immediately clear.
Meanwhile thousands of Peruvians took to the streets in Lima marching against corruption and all those bribed by Odebrecht.
The odds appear stacked against Kuczynski's survival.
The number of lawmakers demanding impeachment on grounds of "permanent moral incapacity" exceeds the vote threshold needed in the 130-seat Congress to pass the motion.
Peru's First Vice President Vizcarra flew back to Lima on Wednesday from Canada, where he is ambassador.
"The president ordered me to return today and here I am, by the president's side," Vizcarra said on arrival.
Political analyst Luis Benavente predicted the center-right head of state's impeachment, telling AFP: "President Kuczynski's luck has run out."
Ecuador's vice president, Jorge Glas, was last week sentenced to six years in prison for taking kickbacks in connection with Odebrecht. Investigations and court cases are taking place in other countries.
The charges against Kuczynski revolve around $5 million he received from Odebrecht between 2004 and 2013.
For part of that period, Kuczynski was economy minister and head of cabinet for then-president Alejandro Toledo, whom Odebrecht said it paid $20 million in kickbacks to win a contract managing a highway project.
Both Kuczynski and the Brazilian company insist the $5 million was for legitimate consulting fees.
However, payment of fictitious "advisory fees" was one technique the Brazilian company has admitted to using to funnel bribes to officials.
And Peru's president, a former Wall Street banker who took office in July 2016, initially denied that he received any money from Odebrecht.
It was only when the Brazilian company revealed the payments that he acknowledged that he had taken them, but denied any wrongdoing.
"I did not lie. I am not corrupt," Kuczynski has repeated.
He has asked the Organization for American States, a cooperative forum for nations across the Americas, to send observers to monitor the moves towards impeachment. The head of the OAS, Luis Almagro, said on Twitter he was looking to dispatch a delegation.
Analysts warn that the political uncertainty hanging over Peru could deal a "strong impact" to its economy, Latin America's seventh-biggest.
Kuczynski is due to make his defense before Congress at 9:00 am (1400 GMT).
The legislature is dominated by opposition deputies, making it seem all but certain that an impeachment vote would be carried. Such a step requires 87 lawmakers to back it -- and 93 have already called for the process to happen.
One opposition party, the Popular Force, had demanded that Kuczynski resign to avoid being impeached.
However suspicions related to Odebrecht are also leveled at the head of the Popular Force, Keiko Fujimori, daughter of a former president imprisoned for corruption and human rights crimes. She was to have given a statement to prosecutors on Wednesday, but postponed the testimony.
Odebrecht, a Latin American powerhouse, had its own dedicated corporate department to manage the bribes it paid.
It ultimately agreed to pay $2.6 billion in fines to the Brazilian, Swiss and US governments for the corrupt practice.