WASHINGTON – Gabe Kapler dug into his bullpen Sunday night, trying to find the right "dude" to record the five outs the Phillies needed to complete a sweep of the Nationals.

Bryce Harper waited on deck and the Phillies were ahead by a run in the eighth inning. A day earlier, the manager rolled with Adam Morgan in the eighth to retire Harper. Kapler instead called Sunday on Seranthony Dominguez, his most trusted arm in the pen. And his most trusted arm came up short in an 8-6 loss.

Harper greeted Dominguez with a double to right to plant the seeds of a rally. Anthony Rendon grounded out into the shift and then Kapler elected to load the bases by intentionally walking Nationals rookie Juan Soto. Kapler said he wanted to take his chances instead against Daniel Murphy, who has been off the disabled list for a week. But Murphy dropped a two-run single over second baseman Cesar Hernandez, who was nearly placed in perfect position to make the catch in shallow right. The Phillies – who once led by four runs – were now trailing.

"I was expecting to face Soto. I wanted to battle with him," Dominguez said. "But I cannot control it. I only focus on what I can control."

Michael A. Taylor added an RBI single and the Nats — once five outs from being swept — had a two-run lead. It was a tough loss that soured what was looking to be the perfect weekend. Sunday was the Phillies' first game on ESPN's "Sunday Night Baseball" since 2013. But their perfect weekend fell a bit short as they watched their bullpen blow a four-run lead.

"We took two of three and won a series. That's four series in a row," said Nick Pivetta, who returned after a 38-minute rain delay to finish a gritty five-inning outing. "We move into the next one and put this one behind us. We're a competitive team here. We did a tremendous job shutting them down the first two days. It is what it is what happened to us today. But It's four series in a row and go into the next one looking for another series win."

Edubray Ramos and Austin Davis struggled in the sixth as the four-run lead dwindled to just one. It looked all but lost in the seventh before Tommy Hunter escaped a jam with a needed double-play. There would be no escaping the eighth. Victor Arano walked his first batter and then picked up an out before Dominguez entered. Five batters later, Kapler walked to the mound to lift his top arm. The Phillies allowed 17 hits, nine of which came against the bullpen.

"We have Luis Garcia, who is not far away, and we have Pat Neshek coming, and I think we have some really good arms who have performed really well," Kapler said. "Ramos has been tremendous. Arano has been tremendous. Seranthony has been really good. Morgan has been better as of late. We have the arms. We have the athletes. We have the horses. Everybody needs to take a small step forward and continue to develop."

The Phillies jumped to their four-run lead on two-run hits from Hoskins, Herrera, and Nick Williams. Hoskins homered in the third, Herrera tripled in the fifth, and Williams homered two batters later. Herrera had two hits and has seven multi-hit games in the last 10 days. Hoskins went 2 for 4 and has five homers since returning from the disabled list.

The loss also came with injuries. Andrew Knapp, who started for the second time in three days, left the game with a bruised right knee. The Phillies do not have another catcher on the 40-man roster. Maikel Franco left the game in the seventh with back tightness after slipping on first base and was replaced at third by Mitch Walding. Kapler said Franco should be able to play on Monday but the third baseman was less certain.

This weekend was the start of a crucial 10-game test as the Phillies play seven games against the Nationals and three against the Yankees. Even with Sunday's loss, they passed the first challenge and have won four straight series. A new challenge awaits on Monday when the Yankees visit South Philly for the first time since 2006.

"There's a number of things that were difficult about tonight's game," Kapler said. "We're going to go back understanding that we took two out of three from the Nationals and that's going to be most important thing."

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