The victory extended the Impact’s winning streak to five games and moved the club to within a point of the fifth-place Unitedfor the final playoff spot in the Eastern Conference.
D.C., which hasn’t won back-to-back games since late May, has three games in hand.
Marco Di Vaio also scored for Montreal (12-13-3).
United fell to 12-9-4 on the season.
Leading 1-0 at halftime, Bernier converted a penalty kick in the 50th minute for his eighth goal of the season.
With the goal, he became the first player since Jamie Moreno in 2009 to score six goals on penalty kicks in a season.
Montreal had chances to add to its lead in the second half, including several from Di Vaio. The Impact forward got a perfect setup from Bernier in the 63rd minute, but the ball sailed over the net. Di Vaio had another chance for his second of the match in the 81st minute, but his shot went wide.
Montreal goalkeeper Troy Perkins preserved his team’s two-goal lead in the 78th minute with a diving save to deny D.C. midfielder Marcelo Saragosa.
Bernier then added his second goal of the game in injury time on a slick finish past United goalkeeper Bill Hamid.
Di Vaio was caught offside several times early in the first half, but got his chance in the 24th minute when Bernier found him behind the D.C. defense. He outwaited Bill Hamid and beat him to the far corner for his third of the season.
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