Donald Trump’s repeated gaffes and slurring during his speeches and public appearances has opened the door for former South Carolina Gov. Nikki Haley (R) to make inroads into cutting into his support as the two battle for the GOP’s 2024 presidential nomination, and one GOP strategist claims his declining mental capacity is fair game. In a column for MSNBC, campaign strategist Susan Del Percio pointed out that the former president has only himself to blame for making his decline front and center as the primary season ramps up and that Haley is doing a good job of raising the alarm that the form…