Agartala, May 13 :    Agartala, May 13:  The ruling Left Front retained its electoral supremacy in Tripura by winning the Banamalipur Assembly constituency for the first time since 1978. The counting of the ballots for the crucial byelection began at 8 am. CPI candidate Prashanta Kopali, who led in all three rounds of the counting, defeated his Trinamul rival Ratan Chakraborty by a margin of 1,393 votes. Chakraborty secured 4,911 votes to forge ahead of Congress candidate Mahitosh Saha. The byelection had been necessitated by the assassination of sitting Congress MLA Madhusudan Saha, brother of Mahitosh Saha. The victory of the CPI candidate made it evident that there was a vertical split in the non-Left votes between the Congress and the Trinamul. The combined votes of the two parties stand at 9,800, 3,496 more than the number secured by Kopali. Senior CPM leader Piyush Nag, who supervised the Left Front's campaign, admitted at the end of the counting, that 'but for the split in anti-Left votes, we would have certainly lost the seat by a big margin'. Senior Trinamul leader and party candidate Ratan Chakraborty attributed his defeat to the 'treacherous backstabbing by Sudhir Ranjan Mazumder who forged a document and campaigned for the Congress candidate and the heavy rainfall on May 10 which led to a fall in polling percentage.' Chakraborty said had the polling percentage been at least 80, he would have won this triangular contest convincingly. He said the Trinamul high command would be informed of Mazumder's 'betrayal and anti-party activities'.