Carlos Vela scored his MLS-leading seventh goal of the season in the 15th minute, ripping a left-footer from the top of the box to give LAFC (5-0-1) a 1-0 lead.
Rossi, from near midfield, played a ball wide to Vela on the left side and then sprinted into the area where he slipped a left-footer inside the near post in the 27th. Rossi first-timed a bending side-netter about five minutes later to make it 3-0 at halftime and capped the scoring in the 76th minute.
Vela and Rossi have more goals (13) this season than any other MLS team.
LAFC has won three in a row and has outscored its opponents 19-5 this season.
D.C. United (3-1-1) played a man down after Wayne Rooney, who came in among the MLS leaders this season in goals (four) and assists (three), was shown a straight red card in the 52nd, after video review, for a dangerous tackle on Rossi.
It was United’s worst loss since a 4-0 decision against the Portland Timbers on Oct. 15, 2017.