Karat made the offer at a meeting of Left-leaning political activists, writers, academics and trade union leaders at the Town Hall to discuss the role of the Left in the ongoing Lok Sabha election.
Those attending the meeting called for "a united fight" to defeat Modi, the Bharatiya Janata Party's prime ministerial candidate, in Varanasi, one of the two Lok Sabha seats he is contesting from.
Karat said all secular forces should reach a consensus on a common candidate against Modi.
"Nominations for the election from Varanasi begin today and there is still time to arrive at a consensus," he added.
Karat made the offer following demands from a large number of the party's local rank and file who feel the CPI-M candidate, Hira Lal, would only end up dividing the secular votes to the benefit of Modi.
Indian People's Front national convener Akhilendra Pratap Singh warned against the emergence of communal forces.
Social activist Teesta Setalwad called for "strategic voting" in favour of the strongest candidate who could defeat Modi.
Janvadi Lekhak Sangh president Doodhnath Singh endorsed her views and said May 12 -- the day Varanasi votes -- "will decide if people of the country want democracy and secularism or dictatorship".
Aam Aadmi Party leader and former Delhi chief minister Arvind Kejriwal has emerged the main challenger to Modi in Varanasi.