Does Sidhu Moosewala's death become the main reason for AAP's loss in Sangrur by-poll elections?

The Mann government has been on the backfoot since the assassination of singer and Congress leader Sidhu Moosewala.

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After the Punjab Assembly elections, the ruling Aam Aadmi Party suffered a major setback in the by-elections to the Sangrur Lok Sabha seat. Shiromani Akali Dal (Amritsar) candidate Simranjit Singh Mann has won the Sangrur seat vacant due to the resignation of CM Bhagwant Mann. He has defeated AAP candidate Gurmail Singh by 5822 votes. This election was being seen as a referendum on the functioning of the AAP government. 

Especially in the matter of law and order, the Aam Aadmi Party government has been under the scanner. The Mann government has been on the backfoot since the assassination of singer and Congress leader Sidhu Moosewala.

"I am grateful to our voters of Sangrur for having elected me as your representative in parliament. I will work hard to ameliorate the sufferings of our farmers, farm-labour, traders and everyone in my constituency" after winning the Sangrur by-election of SAD-A Leader Simranjit Singh Mann said.

Why did Sidhu Moosewala become the centre point?

Sidhu Moose Wala was shot dead on May 29 in Punjab’s Mansa district. The case is now under investigation and the link of gangster Lawrence Bishnoi behind the murder is being probed. However, the late famous Punjabi singer Sidhu Moosewala also supported Simranjit Singh Mann. He was also about to campaign for Mann. It is believed that because of this also the voters have voted fiercely for him. 

After the murder of Moosewala, many questions were raised about the AAP government regarding law and order as his security was withdrawn by the Punjab government a day before the murder. He has a huge fanbase among the youth. Despite the security threat, the AAP government reduced his security. His information was made public. He was murdered the next day which made his fans very angry. Seeing the condition of the father in the rites of Moosewala, the elders also became emotional. He considered it as your failure.

It is wrong at some level to discuss the possible political impact of an event as tragic as the killing of Sidhu Moosewala. However, AAP is indeed facing flak over the matter, especially as Moose Wala's security detail was downgraded by the AAP government a few days before the killing. AAP not only downgraded the security of Moose Wala and 423 other protectees, but they also tweeted out the names showcasing it as the "end of VIP culture". So naturally, AAP has been under attack over the killing.

Had Moose Wala's father Balkaur Singh Sidhu agreed to Punjab Congress president Raja Warring's proposal to become a consensus candidate for all parties from Sangrur, AAP might have had no choice but to support him and he would have probably won. But he refused and understandably so.
So it is somehow very clear his death factor had played very well in the poll. 

However, there is another, significant claimant to that vote – Shiromani Akali Dal (Amritsar) president and former Sangrur MP (1999-2004) Simranjit Singh Mann who recently won the Sangrur seat. Moosewala might have been a Congress leader but he respected Simranjit Singh Mann. Moose Wala may not have entirely been in SS Mann's ideological corner but on many issues, he may have been closer to him than the Congress.