The Nationals came back from a 6-0 deficit to the Royals on Friday.
They've now come back after trailing 4-0 this afternoon and battled to tie this game 4-4 in the seventh.
The three runs all came with two outs, and they came in the span of three pitches.
Span smoked a first-pitch Ervin Santana changeup for his third homer of the season, Ryan Zimmerman followed with a first-pitch single to left, and Harper jumped all over a first-pitch Santana fastball for his 18th homer of the year.
Harper's homer came after he fouled a ball off his left leg in his previous at-bat, which left him crumpled in a heap for a minute. Harper was looked at by trainer Lee Kuntz, but he stayed in, and is now 2-for-2 since getting dinged by that foul ball.
Santana is now out of the game, and he won't be notching his ninth win of the season today.
Dan Haren, meanwhile, has done well to right the ship after a rough first inning, in which he allowed four runs and two homers. Since then, the right hander has allowed just four hits and no walks over five scoreless frames.
It's pretty clichÃ© to say, but the Nats just haven't been giving in lately. They've battled despite blown leads and early deficits, and now they have a shot at their sixth straight win and a sweep over the Royals if they can finish the job.
Update: The Nats now trail 6-4 heading to the ninth thanks to a complete loss of defensive fundamentals in the eighth.
Craig Stammen retired the first two hitters he saw in relief but then failed to cover first base on a ground ball at Adam LaRoche. LaRoche couldn't handle the grounder initially, and then when he corralled the ball and tried to flip to first, he saw Stammen wasn't there. Billy Butler reached to extend the inning.
Two batters later, Salvador Perez hit a grounder in the hole at short with the bases loaded. Ian Desmond got to the ball but, like LaRoche, couldn't handle it cleanly. His only play at that point was to try and get a force at third, but Zimmerman didn't cover the bag. Desmond got a throw off to Zimmerman, but the third baseman's lunging tag attempt couldn't get Butler in time. That allowed a run to score and the inning to continue, and Stammen then allowed an RBI single to David Lough.
If the Nats can't come back in the ninth, they'll be kicking themselves for their defensive miscues there in the eighth.
Update II: That'll do it. The Nats brought the go-ahead run to the plate in the ninth after Harper and Jayson Werth reached, but Desmond struck out to bring an end to a 6-4 loss.
The Nats can't extend their winning streak to a season high six games. They drop back to .500 at 65-65 and finish their road trip, going 6-4.
A day off comes tomorrow, and then 19 straight against the Marlins, Mets and Phillies await.