BRIGHTON, England, Sept 28 (Reuters) - Labour leader Keir Starmer will set out his vision for Britain on Wednesday, promising to show the party is "back in business" by reversing a lurch to the left to try to win back voters lost to Prime Minister Boris Johnson"s Conservatives. Britain"s main opposition party has not won an election in more than 15 years, but Starmer hopes a series of crises and missteps by the governing Conservatives offers him the chance to show Labour as a viable and attractive alternative. Against the backdrop of COVID-19, Labour hopes to replicate some of the centre-left"s successes elsewhere in Europe by focusing on the importance - and often, also, the poor pay - of key workers in health, social care and other essential sectors. But critics and even some in his party question whether the former human rights lawyer has what it takes to unite a deeply divided party or challenge Johnson"s ability to connect with voters. "The questions we face in Britain today, are big ones. How we emerge from the biggest pandemic in a century. How we make our living in a competitive world. The climate crisis. Our relationship with Europe. The future of our union," he will tell delegates at the annual party conference in the English seaside town of Brighton, according to excerpts of his speech. "These are big issues. But our politics is so small. So our politics needs to grow to meet the scale of the challenge." Starmer, 59, wants to underline that the party is under new management after Labour was all but consumed by a civil war between its two wings under leftist former leader Jeremy Corbyn. "MORE OPTIMISTIC" Labour is closing the gap with the Conservatives in opinion polls and a spokesperson said the party was keen to show it was a competent alternative for a Britain beset by fuel shortages, empty shelves in some stores and soaring energy prices. The party also wants to win back traditional supporters in northern and central England who voted for Johnson and his party at a 2019 election when the prime minister adopted many of Labour"s more centrist policies. "Keir"s speech will be noticeably different from what you"ve heard from Labour in recent years. It will be more optimistic, more focused on the future, more outward-looking," a Labour source said. "The speech will be a demonstration of the way the Labour Party has changed. It will be a clear indication that Labour will never again go into an election with a manifesto that isn’t a serious plan for government."