Former President Barack Obama aims to rally Black men around Vice President Kamala Harris' presidential campaign.Jeff Swensen/Getty ImagesObama admonished some Black men for seeking "excuses" not to back a female presidential candidate.Some took issue with the message, arguing that Black male voters aren't Harris' biggest challenge.But any critical erosion in Black support could prove costly for Democrats in November.Former President Barack Obama this week hit the campaign trail for Vice President Kamala Harris, part of a concerted effort to rally support around the Democratic presidential nominee with less than a month remaining before the election.Obama has long been known for his soaring oratory, which helped propel him to the White House in 2008.