President Trump will ask Congress on Monday for $8.6 billion in additional funding to build a wall along the United States border with Mexico, a person familiar with the details said on Sunday. The request, which will come as part of Mr. Trump’s fiscal 2020 budget proposal, is certain to reignite a conflict with Democrats that led to a record-long government shutdown this year. Mr. Trump had previously requested $5.7 billion to build a wall but was rebuffed by both Democrats and Republicans, who approved a spending bill that did not include the funding. That resulted in Mr. Trump declaring...