BROWNSVILLE, Texas (AP) — Three hundred miles apart, on the bank of the same winding Rio Grande, President Joe Biden and GOP presidential front-runner Donald Trump on Thursday surveyed the U.S.-Mexico border. They then tussled from a distance over who was to blame for the nation’s broken immigration system and how to fix it. Immigration has emerged as a central issue in the 2024 presidential campaign, which is widely expected to be a Biden-Trump rematch, and each is seeking to use the border problems to their political advantage.