"People in India forget that the president of BJP is a murder accused. That's the truth. The party that talks about honesty, decency, has a person who has been accused of murder as president," the Congress president said, a reference to the accusation that Mr Shah once faced about his alleged involvement in the extra-judicial killing of a petty criminal, Sohrabuddin Sheikh, in 2005.
Mr Shah, who is acknowledged as PM Modi's closest advisor, was discharged in the case investigated by the CBI in 2014. Two years later, the Supreme Court rejected demands that he should be investigated again.
The recent row around the death of the trial court's judge BH Loya who had heard the fake encounter case was also widely seen as targeting Mr Shah, 53.
"Amit Shah has been accused of murder, right? The Supreme Court has mentioned the (judge) Loya case. So I don't think Amit Shah has a lot of credibility.... I mean, let's look into his background. Let's see what he is responsible for. Let's see how he does politics. Also, don't forget he is a murder accused. This is fundamental," Mr Gandhi told reporters.
Amit Shah, who has been camping in Karnataka for days, is widely credited with playing the top role in PM Modi's stunning victory in the 2014 election and praised for his political acumen that helped the BJP come to power in most of the 21 states where it is in power on its own or in an alliance.
The BJP is making an all-out effort to wrest power in the only state in south India where it had formed government. A win in Karnataka will also demonstrate that the BJP's winning streak continues unabated. For the Congress, the Karnataka is one of the four states where the party is in power.
The campaign has been bitter and shrill, with allegations of communal politics and political murders and tit-for-tat charges of large-scale corruption.
Mr Gandhi also questioned Prime Minister Narendra Modi's for picking a "corrupt person" as the chief ministerial candidate in Karnataka.
"We are asking the Prime Minister to please explain to the people of Karnataka why he has chosen a corrupt person with a record of being in jail as Chief Ministerial candidate. People of Karnataka want to know why Modi could not find a non-corrupt candidate," Mr Gandhi said.