Donald Trump has had fierce competition in the polls ever since Kamala Harris ascended to the top of the Democratic ticket, but a presidential historian said he might have one last chance to regain momentum. New polling shows the vice president with a five-point lead over the ex-president with a little over six weeks until Election Day, and historian Douglas Brinkley told MSNBC’s “Morning Joe” that Trump is running out of chances to turn things around. “Donald Trump is a usurper of democracy, he is a wrecking ball,” Brinkley said. “You’ve been talking, rightfully reminding people of Jan. 6 but…