evidence: President Joe Biden has tapped Vice President Kamala Harris to lead the White House effort to tackle the migration challenge at the U.S. southern border and work with Central American nations to address root causes of the problem. [...] Biden, who has faced stiff criticism from Republicans over the increased flow of migrants, is hoping to show Americans he's taking the situation at the border seriously. He also wants to prevent the growing humanitarian and political challenge from overshadowing his administration's ambitious legislative agenda. [...] Biden made the announcement as a delegation of White House officials and members of Congress was traveled to the southern border on Wednesday to tour a facility in Carrizo Springs, Texas, where more than 750 migrant teenagers are being held.