SEOUL (Reuters) -South Korea's left-wing presidential frontrunner Lee Jae-myung proposed on Friday to amend the constitution to make it more difficult to impose martial law, aiming to prevent political crises like the one that erupted last year. As candidates entered their final stretch before the snap presidential election on June 3, Lee and his Democratic Party urged South Koreans to come out to vote to end the political turmoil that has gripped the country since ousted conservative leader Yoon Suk Yeol's short-lived martial law declaration in December.