BUENOS AIRES (Reuters) - The government of Argentine libertarian President Javier Milei was racing to secure votes in Congress for its key 'omnibus' reform bill that entered the lower of chamber on Wednesday, even as the left-leaning opposition pledged to oppose it.

       The bill, ranging from economic policy to privatization of state entities, is one the main planks in Milei's reform push to tackle the South American country's worst economic crisis in decades, with inflation over 200% and state coffers running dry.

       It marks his first major test since taking office in December after a shock election win for the economist who made his name as an acid-tongued TV pundit and campaigned with a chainsaw pledging to slash spending and the size of the state.

       Milei faces a major challenge to push the bill through with his coalition only having a minority in both chambers, which means he has to win over allies. His government yanked a divisive fiscal section from the bill last week to boost support.

       "Today, politicians have the chance to start reversing the damage they've caused the Argentine people," Milei's office said in a post on Wednesday, urging lawmakers to support the bill as an expected mammoth debate by legislators began.

       In a sign of the challenge ahead, however, the main Peronist opposition bloc Union por la Patria, which is the largest single grouping in Congress, said it would reject the bill, posting a picture with a slogan "No to the Omnibus bill" on X, formerly Twitter.

       "We reject the Omnibus bill because it puts fuel in Milei's chainsaw to hurt the daily lives of Argentines," wrote Peronist politician and former foreign minister Santiago Cafiero, a reference to his austerity plans to undo a deep fiscal deficit.

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       If the law is approved in the lower chamber - a debate that will likely extend beyond Wednesday - it moves next week to the Senate. Investors are watching closely, with hopes that the pared back version will pass, which would bolster local markets.

       "The Omnibus bill now has a better chance of being approved in Congress," said Fernando Sedano of Morgan Stanley.

       "We continue to consider that the approval of radical reforms is positive for the medium-term narrative, even with a watered down version of the original bill."

       (Reporting by Walter Bianchi; Editing Adam Jourdan and Bill Berkrot)