Vice President Kamala Harris’ presidential campaign came out Tuesday with a fiery ad slamming former President Donald Trump on an issue that matters deeply to him: the border. And more specifically, how he worked behind the scenes to tank Republican support for a bipartisan bill that would have helped shore up border security, even as he attacked President Joe Biden for doing nothing about a surge in migrants. “On the border, the choice is simple,” said the ad’s narrator. “Kamala Harris supports increasing the number of border patrol agents. Donald Trump blocked a bill to increase the number o…