He didn't have much luck, however, facing their lineup two weeks ago.
Seeking his first victory in six weeks, the struggling Haren tries to send the visiting Rockies to a sixth straight defeat Saturday.
Washington (37-36) has had a difficult time replicating the success it enjoyed last season when it won its first division title in franchise history.
The Nationals opened June with nine losses in 15 games before stringing together three straight wins to pull within five of NL East-leading Atlanta.
"We're still not where we need to be," manager Davey Johnson said, "but we're going to get there."
Trips to the disabled list for Bryce Harper, Stephen Strasburg, Ryan Zimmerman and Jayson Werth have factored in Washington's inconsistent start, as has an offense that is averaging 3.5 runs - 1.0 less than a year ago.
And then there's Haren (4-8, 5.72 ERA).
The right-hander has an 8.40 ERA in three starts this month and is 0-5 in seven outings since beating Detroit on May 9, with the Nationals losing each time. The three-time All-Star hasn't gone eight straight starts without a win since a career-worst 10-start drought in 2010.
"We have faith in him," Johnson said. "He'll be fine."
Haren's biggest problem is he's allowed an NL-worst 18 homers.
Carlos Gonzalez and Tyler Colvin took him deep in an 8-3 loss at Coors Field on June 11. Haren gave up five runs and eight hits in five innings before yielding four runs in six innings while not receiving a decision in a 5-4 defeat in Philadelphia on Monday.
Since 2010, Gonzalez is 7 for 13 with four homers and a double against Haren, and Colvin is 3 for 5 with a pair of home runs. Neither had any success at the plate in the first two games of this series, though, going a combined 0 for 13 with seven strikeouts.
Colorado (37-38) has dropped eight of 10 and scored seven runs with a .183 average during its season-high five-game skid.
"There's no secret formula," manager Walt Weiss said. "You go up there, take good at-bats, take some timely hits. That's all you can do."
Ian Desmond has found his power stroke for the Nationals, homering in each of his last three games. He is 9 for 17 with seven RBIs in five games against the Rockies this season and went 0 for 1 with a walk two weeks ago against Jhoulys Chacin (5-3, 4.26), who is looking to beat Haren for a second time.
After allowing two runs in seven innings against the Nationals on June 11, Chacin permitted two runs and six hits and came within one out of a complete game in Sunday's 5-2 win over Philadelphia. The right-hander walked none and threw 69 of 86 pitches for strikes.
This will be Chacin's first outing away from Coors Field since May 27, and he has a 1.88 ERA in nine road starts since August.
He hasn't pitched in the nation's capital since 2011 and is 2-1 with a 3.18 ERA in his last three starts in the series.
Zimmerman doubled off Chacin earlier this month. He's 5 for 10 with two homers in their matchups.