WASHINGTON, March 11 (Reuters) - Several U.S. lawmakers have written to President Barack Obama urging him to follow up on campaign statements and label the 1915 massacre of Armenians as genocide. The pressure on Obama comes ahead of an expected presidential trip to Turkey, which has warned that such declarations by the United States would damage relations. Turkey denies that up to 1.5 million Armenians suffered genocide at the hands of Ottoman Turks during World War One. Turkey accepts many Armenians were killed, but denies they were victims of a systematic genocide. Ronald Reagan was the only U.S. president to...