Tied at 4, Brothers took over for right-hander Brooks Brown with runners on first and second. Brothers was down 2-0 in the count before trying go down and away with a fastball to LaRoche.
Instead, the pitch crossed over the inner third of the plate, otherwise known as LaRoche's wheelhouse. The result was LaRoche's first homer off a left-handed pitcher since May 28, 2013. It was also Brothers' 14th, 15th and 16th earned runs allowed over his last 18 2/3 innings, a telling stat for a reliever who registered a 1.74 ERA last year.
"Physically, he feels very good, very strong," manager Walt Weiss said. "I think anybody that struggles in this league, his confidence becomes a factor and you have to battle through that. But he believes he's going to succeed every time he goes out there. ... Sometimes it just takes a run of success to get over that hump."
Weiss has shared that Brothers has been working through some mechanical issues all season long. Brothers, on the other hand, wasn't in the mood for any mitigation.
"It think it's a grind," Brothers said. "I'm going to answer the bell whenever it rings. I'm not backing out for anything, I feel like this game will teach you a lot if you let it. ... Obviously, it wasn't the results I wanted tonight and I didn't do many things right. But I'll never back away from it, ever. Or make an excuse either."
Down Tulowitzki and Justin Morneau (neck), the Rockies' offense got to All-Star right-hander Jordan Zimmermann for his season-high in home runs allowed -- two. Yet their four runs don't seem to be nearly enough considering they outhit the Nationals 13-11 with three doubles and two triples.
"We had some missed opportunities, no doubt," said Weiss, whose team went 2-for-15 with runners in scoring position. "Again, when you're facing a staff like this and you get some opportunities early, if you don't capitalize, they usually come back to haunt you."
One day after singling in his first Major League at-bat, Ben Paulsen doubled in his first plate appearance Tuesday to start the second. DJ LeMahieu's triple then gave Paulsen his first Major League run and the Rockies a 1-0 lead.
Nolan Arenado's first homer since May 5 put the Rockies up two more after Josh Rutledge led off the third with another triple.
"I was just missing over the middle," Zimmermann said. "I'd get two strikes and then miss over the middle, and it seemed to be a double or a triple every time. It was a real battle tonight. It seems like I had my hands full every inning."
Operating with the largest lead he's had since his Major League debut on June 25, left-hander Yohan Flande's pitch-to-contact mentality got the best of him in his final two frames.
Ian Desmond's double -- his seventh hit in seven at-bats this series -- put runners on the corners following Bryce Harper's leadoff walk. A Wilson Ramos groundout and a Zimmermann sacrifice fly to center then put the Nationals' first two runs on the board.
After Corey Dickerson's second-deck, solo shot, the Nationals put runners on the corners again, chasing Flande with one out in the sixth.
Right-hander Chad Bettis took over having surrendered four runs over his last 1 2/3 innings pitched and the results weren't too different Tuesday.
Ryan Zimmerman exited with a leg injury on his ensuing groundout, but not before it plated Anthony Rendon. After the next two batters reached, Ramos dug the first pitch Bettis sent his way out of the dirt with the bases loaded, leaving the sure-handed LeMahieu to make a tough play a second. He couldn't and Nationals tied the game at 4.
Flande looked slightly sharper than Zimmermann in his return to the rotation, but was unable to make it past the first out of the sixth for the fourth time in four Major League starts. He finished his 5 1/3 innings pitched with five hits, four earned runs and two walks without registering a strikeout.
The Rockies loss drops their record to 6-25 since sweeping the Giants in San Francisco last month. With the Rangers also losing in extra innings to the Yankees Tuesday, the Rockies remained tied for the worst record in baseball at 40-60.