LIMA, Nov 19 (Reuters) - Peru"s Keiko Fujimori, the head of the largest opposition bloc, said on Friday her Popular Force party would back a motion being prepared to impeach socialist President Pedro Castillo. The basis for the challenge against Castillo, alleging he is morally unfit for office, is tenuous and its chance of success unclear. Fujimori narrowly lost the presidential race against Castillo earlier this year. But Peru"s fragmented Congress, which overall leans towards the right, has a history of controversial impeachments and left-wing Castillo"s popularity has declined since he came to power in July. "At Popular Force, we believe that this government has been demonstrating a permanent incapacity to lead the country," said Fujimori in a tweet. The impeachment motion has been drafted but it has yet to be formally introduced or voted on, apparently because its backers are still working to gather enough support Castillo has been in power for just four months, a period which has seen a number of key ministers come and go, and concerns over political instability has driven the sol currency to record lows. He has also faced a hostile Congress and Castillo"s own party, the Marxist-Leninist Free Peru, has broken ranks with him on accusations that his administration had taken a right-wing shift in a bid to appease investors and the opposition. Lawmakers need 52 votes out of the 130-member legislature in order to begin the impeachment process against Castillo. Then they would need 87 votes to oust him. Popular Force has 24 votes. Individual members of two other parties commanding 20 additional votes have also spoken in favor of impeachment. Left-wing forces only command 42 votes in Congress, meaning that a super-majority of lawmakers belong to the center or the right. Peruvian presidents are notoriously easy to impeach and Castillo is Peru"s fifth president in five years. In 2018, Pedro Pablo Kuczynski resigned the presidency minutes before an impeachment vote he was sure to lose, while in 2020 Martin Vizcarra was ousted in an impeachment vote. The impeachment effort also follows an unsuccessful bid in neighboring Chile to oust moderate President Sebastian Pinera. The vote passed in the country"s lower-chamber but failed in the Senate earlier this week.