WASHINGTON — The Democratic and Republican presidential candidates are coming to terms with the cold mathematical reality of chasing delegates ahead of their nominating conventions, with front-runners Hillary Clinton and Donald Trump driving for challenge-proof majorities against rivals who won’t go away. For Trump, who remains well short of the 1,237 delegates he needs to clinch the GOP nod, that means his campaign focuses on developing a delegate-centered strategy akin to the one that rival Ted Cruz has pursued for months. “A more traditional approach is needed and Donald Trump recognizes that,” Paul Manafort, Trump’s new delegate chief, said Sunday on NBC’s “Meet the Press.” Even so, the billionaire developer later in the day complained that the system is “corrupt” and that it’s unfair that “it’s not right” that the person who wins the most votes may not be the nominee.