PORTLAND, Maine — Former Vice President Joe Biden said in Maine on Wednesday night that he thinks the U.S. will “see a significant change” in the next election, taking a few shots at President Donald Trump without naming him while demurring on his 2020 plans. The Democrat’s visit to Portland’s Merrill Auditorium was part of a tour in support of his recent memoir, “Promise Me, Dad: A Year of Hope, Hardship and Purpose,” which centers on the death of his son, former Delaware Attorney General Beau Biden, from cancer in 2015. Biden, 75, who served for 36 years as a senator from Delaware before eight years as Barack Obama’s vice president, has not ruled out a run against the Republican president in 2020.