With the New York Islanders losing 2-1 to the Toronto Maple Leafs, Washington (47-25-8) would have clinched first with a win. The Capitals lead the second-place Islanders by three points with two games remaining for each.
"Obviously, it would have been nice to clinch it, but we've got two more chances," center Nicklas Backstrom said. "Hopefully, we can get it done."
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Washington scored three straight goals in the third period to get to within one, but the Panthers held on to complete their first season series sweep of the Capitals, following victories at Capital One Arena on Oct. 19 (6-5 in the shootout) and Feb. 9 (5-4 in overtime).
Backstrom, Evgeny Kuznetsov and Jakub Vrana scored for Washington, which lost for the first time in five games. Pheonix Copley made 29 saves.
"No one quit," Capitals coach Todd Reirden said. "That's too big of a lead to spot anybody, even with the talent that we have in our room. When things aren't going like we'd like them to early in the game, we gotta stop it at two. We can't let it get to four. That's too difficult to come back from. At the end, I'm proud that our guys didn't stop. We didn't miss by much on a couple of those chances to make it 4-4."