Prospective running mates have received vetting materials as the race for the No. 2 spot on Trump's ticket narrows to three or four candidates, sources tell NBC News.

Trump’s search, according to one source, is heavily concentrated on four top prospects: North Dakota Gov. Doug Burgum and Sens. Marco Rubio of Florida, Tim Scott of South Carolina and JD Vance of Ohio. Another source described a three-way competition involving Burgum, Rubio and Vance.