WASHINGTON—Kamala Harris is making abortion rights central to her candidacy for president as Republicans struggle to articulate a winning message on the issue. In contrast with President Biden, who was reluctant to say the word abortion, the vice president has campaigned aggressively on it since the 2022 Supreme Court ruling that eliminated the constitutional right to the procedure. At the White House, she has met with abortion providers and women who have had abortions. Earlier this year, Harris was believed to be the first president or vice president to visit an abortion clinic. After locking up the support to be...