WASHINGTON — As they won another series against a first-place team Sunday, with a blend of power, strong starting pitching and serviceable relief work, the Rockies were left to wonder what could have been. Colorado’s 5-3 victory gave the Rockies their second straight series win over the Nationals. It came on the heels over recent such triumphs over the Chicago Cubs, Los Angeles Dodgers and Baltimore Orioles, all teams currently on track to play postseason baseball in October. But the goodwill those wins have created, and the ground in the standings they’ve helped make up, have too routinely been followed by veritable flops against teams further down the major-league totem pole than the Rockies.