Defamation charges can be brought against political leaders in India under the following circumstances:

• If the statements made by the politician are false or baseless and harm the reputation of the person against whom it is made.
• If the statements are made with the intention of causing harm to the person's reputation or are made recklessly without any regard for the truth.
• If the statements are made in a public space or in front of a large audience, where they are likely to be widely disseminated and cause harm to the person's reputation.

In the specific case of Congress leaders Rahul Gandhi and Siddaramaiah being summoned in a defamation case filed by BJP, the legal grounds for bringing such charges would be based on the allegation that the leaders made false and defamatory statements about the BJP, which were intended to harm its reputation and were made in a public space. It will ultimately be up to the court to decide whether the charges are proven or not.