The Pittsburgh Pirates have matched their longest losing streak of 2014 after two one-run losses in the nation's capital.
LaRoche has a favorable matchup against Pirates starter Edinson Volquez, who will be up against 12-game winner Doug Fister on Sunday as the Nationals seek a three-game sweep.
Washington (68-53) has posted victories of 5-4 on Friday and 4-3 on Saturday to match its longest win streak.
LaRoche has six RBIs in his last three games, going 3 for 7 with four in this series. He went 2 for 4 with two RBIs Friday before his only hit Saturday was the biggest of the game.
The Nationals trailed 3-0 going into the bottom of the eighth inning before Kevin Frandsen had an RBI single and LaRoche came through with a tying two-run homer with two outs.
Washington prevailed on Wilson Ramos' RBI ground-rule double in the bottom of the ninth.
''When you're on a streak, when you've got a little lead in the division, it's easier to go relax and play,'' said LaRoche, ''and not get too uptight late in games, regardless (of whether) you're down or not.''
LaRoche is 6 for 16 with a homer and two doubles off Volquez (10-7, 3.67 ERA), who has won six of his last seven decisions while Fister (12-3, 2.34) is 6-1 with a 1.69 ERA in his last seven starts.
Pittsburgh (64-59) has won all five of Volquez's road starts since the beginning of June, with the right-hander going 3-0 with a 2.53 ERA. He enters off consecutive home wins, yielding two runs in six innings of Tuesday's 4-2 victory over Detroit.
"He's been fun to watch," manager Clint Hurdle said. "From the time we got him he showed up wanting to get better and wanting to learn."
Volquez will pitch in Washington for the first time. He is 2-2 with a 5.47 ERA in five outings against the Nationals, beating them May 22 by allowing one run in six innings of a 3-1 victory.
Fister hasn't yielded an earned run in his last 15 1-3 innings. He pitched seven scoreless innings in Tuesday's 7-1 win over the New York Mets but didn't want any of the credit.
"This is a team game," Fister told MLB's official website. "I can only do so much. I can only release the ball. After that, everybody else has control. That's what we do. We're a team here."
The right-hander is 1-1 with a 1.47 ERA in three career starts against the Pirates, allowing one run in 5 1-3 innings to earn a 5-2 road win May 25. Neil Walker is 4 for 8 against him.
Walker is expected to start Sunday after not doing so Saturday due to ongoing back problems.
Pittsburgh has fallen out of a wild-card spot thanks to this losing streak. The Pirates last dropped five straight Sept. 18-22, 2012, and are seeking to avoid losing six in a row on the road for the first time since a 10-game slide two years ago.
''It's never good for the club, especially the position we're in right now, pushing hard,'' said reliever Tony Watson, who served up LaRoche's homer Saturday.