Donald Trump may have a card up his sleeve for an “October surprise,” and J.D. Vance should be worried, a conservative columnist said Saturday. During an appearance on MSNBC over the weekend, columnist Jennifer Rubin — a loyal conservative until Donald Trump came along — was asked about the impact Robert F. Kennedy Jr.’s recent Trump endorsement might have on the presidential race. Specifically, the host said, “This average of national polls we will put this up here on the screen. You can see there, before Vice President Harris took over the top of the ticket, with Kennedy in the race, Trump w…