Portland (8-3-6, 30 points) has eight wins and four ties in its last dozen games.
LAFC (10-4-5, 35) improved to 5-0-4 at home in its first season and has four wins and two ties in the past six games.
Portland's Jeff Attinella earned his fifth clean sheet with six saves, including one on a snap header by Carlos Vela in the 93rd minute.
In the 31st minute, Adama Diomande missed giving LAFC the lead. He took a scooped pass from Joao Moutinho over his shoulder and one-touched a shot to the right post, but Attinella was there to make the save.
Portland was inches from scoring early in the second half only to see a strike from Samuel Armenteros hit the post.
The teams played the first of two matches against each other in a span of four days at Banc of California Stadium. They meet Wednesday in a U.S. Open Cup quarterfinal match.
LAFC plays at Minnesota on July 22.
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