WASHINGTON - Ian Stewart and Seth Smith hit three-run homers, Aaron Cook struggled through five innings for his 15th victory and the Colorado Rockies beat Washington 7-2 on Sunday, handing the Nationals their season-high 10th straight loss.

It's the longest skid for the Nationals since they moved to Washington before the 2005 season, and the franchise's worst since the Montreal Expos dropped 11 games in a row from June 23-July 4, 1991.

With 37 regular-season games remaining, the Nationals need to go 19-18 to avoid the club's first 100-loss campaign since the Expos went 55-107 in 1976.

Cook (15-8), whose scheduled Thursday start was pushed back because of lower back fatigue and tightness, won for the first time in three August outings. The right-hander allowed two runs on seven hits, walked three, struck out four and averted his first three-game losing streak of the season.

Brad Hawpe added a solo homer for Colorado, which swept the weekend series and has won four of five overall.

Washington grabbed a 1-0 lead in the second when Ronnie Belliard led off with a single, moved to third on Ryan Langerhans' double to left and scored on Pete Orr's grounder to first.

The Rockies, who had a streak of 10 straight games with a home run snapped in a 13-6 victory Saturday night, went ahead for good on Stewart's three-run shot in the fourth. He leads all NL rookies with 27 RBIs since the All-Star break.

Chris Iannetta reached when Orr booted his grounder behind second base, Troy Tulowitzki drew a walk and Stewart hammered a 2-2 pitch from Odalis Perez to right-center field for his eighth homer.

The Nationals got within 3-2 with one out in the fifth when Emilio Bonifacio walked, Willie Harris singled and Lastings Milledge bounced a ground-rule double into the left-field bullpen.

Perez (5-9) went six innings, giving up three runs - two earned - on four hits. He walked four and fanned three.

Hawpe hit his 21st homer off Charlie Manning leading off the eighth. Jesus Colome relieved and hit Iannetta with a pitch, issued a one-out walk to Stewart and then surrendered pinch-hitter Smith's three-run blast.