Sources say that he has asked the party branch in Karnataka to identify nearly two dozen districts where it is strong and is likely to perform well.
The sources, who are close to the chief minister, say that he is determined not to rally for the BJP in "unsafe" constituencies because if the party loses there, his image as the BJP's most popular leader and resolute vote-getter will be dented.
He will start his campaign on April 28, sources said.
Mr Modi is vying to be declared the BJP's prime ministerial nominee. An important ally of the BJP, Nitish Kumar, has said he will end his partnership with the BJP if that happens because Mr Modi is tainted by the Gujarat communal riots of 2002, which took place during his first term in office.
Criticism from Mr Kumar and his Janata Dal (United), as well as detractors within the BJP, come amid a massive surge in Mr
Modi's popularity which was powered by his third consecutive re-election in Gujarat in December.
Party cadres have been urging BJP's top leaders to declare Mr Modi its presumptive prime minister.
In Karnataka, Mr Modi plans to tread carefully because the BJP is seeking a second round in office after a first term that was littered with corruption scandals and dissent among its senior most leaders.