"JD(S), they have to make it clear whether they are the BJP's B team or not," Mr Gandhi said at a public meeting here in the Vokkaliga belt of the old Mysuru region, considered a JD(S) stronghold.
"They (JD-S) have to make it clear whether they are supporting the BJP, if yes, why? The JD(S) stands for 'Janata Dal Sangh Parivar'," he added.
The Congress president, who is in the fourth round of campaigning in poll-bound Karnataka, is currently touring the old Mysuru region, where the Vokkaliga community has a significant presence. The JD(S) enjoys the support of the Vokkaligas.
The Assembly election is due in Karnataka by April-May.
Terming employment generation the "biggest problem" in the country, Mr Gandhi attacked the BJP-led central government for "not" fulfilling the promises it had made to the people such as job creation and addressing the issue of farmers' distress.
Mr Gandhi even credited the chief minister with running a "clean government".
Raising the Nirav Modi issue and alleging that the diamantaire had run away with Rs 22,000 crore, the Congress president questioned Prime Minister Narendra Modi's silence on the matter.
"What kind of chowkidari (watchman's job) is Narendra Modi doing," he asked.
Earlier in the day, Mr Gandhi visited Beluru in Hassan district to pay homage to Congress lawmaker Y N Rudresh Gowda, who passed away Saturday.
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