"He will campaign at all places. He will campaign in the whole country," Singh told reporters in reply to a question whether Modi will campaign in Karnataka.
BJP seeks to retain power in the state where the party is buffeted by internal differences after B S Yeddyurappa resigned from the Chief Minister's post following corruption allegations.
On whether Modi's name has also been proposed as the Prime Ministerial candidate from BJP, Singh said "I am the party President."
"No. Whatever happens and whenever it happens, we will sit together," he said when asked whether BJP is planning to snap ties with JD(U) after Karnataka elections.
Reacting on the serial bomb blast in Boston in the US, Singh said the "US should make a joint forum of the countries which are affected of terrorism to fight terrorism.