BERLIN—Centrist Armin Laschet positioned himself on Saturday as the man to heal divisions among Angela Merkel’s Christian Democrats after they chose him to lead the party, putting him in pole position to succeed her as Germany’s chancellor. Laschet, premier of the country’s most populous state and the self-styled Merkel continuity candidate, beat arch-conservative Friedrich Merz in a ballot of CDU party delegates. Merkel, Europe’s predominant politician and a consistent winner with German voters since taking office in 2005, has said she will not run for chancellor again in September’s federal election. Since she stepped down as CDU leader in Dec. 2018, the party has struggled to find a suitable successor. In choosing Laschet, premier of the Netherlands-sized state of North Rhine-Westphalia, delegates opted for a candidate more palatable to the left-leaning Greens party, second behind the conservatives in opinion polls and seen as a potential coalition partner come September. But …