By Jason Lange and James Oliphant WASHINGTON (Reuters) -A looming election rematch next year between U.S. President Joe Biden and his predecessor Donald Trump would be closely fought, a new Reuters/Ipsos poll found, with both candidates saddled with profound vulnerabilities that could cost them the White House. Biden, an 81-year-old Democrat, continues to be plagued by voters’ doubts about the strength of the economy, as well as concerns about the security of the U.S.-Mexico border and worries about crime. Republican former President Trump, 77, faces his own worries, including four criminal tr…