Jaroslaw Kaczynski's socially-conservative Law and Justice party lost Sunday's early elections to pro-business Civic Platform, which also has conservative views on many social issues such as abortion and gay rights. Kaczynski said he will step down during new parliament's first session Nov. 5.
"We will be tough in watching whether Civic Platform party keeps their promises," Kaczynski was quoted as saying in the Rzeczpospolita daily.
"They made many," he said, naming the promise to quickly increase earnings, build much-needed highways and houses and lure home hundreds of thousands of Poles who have left for England and Ireland to work.
In the interview, Kaczynski blamed his defeat on - among other things - a TV debate with Tusk to which he went "tired, a little sick and unprepared" and on a hostile campaign in some media.
Tusk, 50, is in coalition talks with the small Polish People's Party, also known as the Polish Peasants' Party.
His close aide leading the talks, Grzegorz Schetyna, said they should be completed by Nov. 1.
Kaczynski called early elections in a failed bid to strengthen his minority government, after a shaky coalition collapsed in the summer.