Lots of people wondered why on earth Donald Trump accepted an invitation to speak at the Libertarian Party convention in Washington over the weekend, where he was booed and jeered as he urged attendees to support him. The answer is pretty simple: He needs votes. Some would say President Biden also needs votes, yet he didn’t accept an invitation to speak to the group. And sure, Biden, too, needs votes. The difference is that Biden, who would likely have been met with the same hostile reaction, has lots of room to grow and nail down support among his traditional Democratic base and among indepen…