Donald Trump’s calculus for choosing J.D. Vance as his running mate might already be outdated, according to MSNBC’s John Heilemann. The former president has been banking on a rematch with President Joe Biden, who dropped his own re-election campaign Sunday over widespread concerns about his age. And the political analyst told “Morning Joe” that Trump might already regret doubling down on MAGA extremism as Vice President Kamala Harris emerges as the likely Democratic nominee. “Your core conviction is this race is going to be close,” Heilemann said. “Donald Trump and his team’s core conviction i…