WASHINGTON -- They're not saying "boooooo." They're saying "Dooooooo."

During the ninth inning of Thursday night's 3-2 comeback win over the Marlins, recently acquired closer Sean Doolittle was showered with a rousing chorus of "Doos" from the crowd in D.C. The reaction was a far cry from the first half of the season, when Nationals fans regularly rained boos down on the Washington bullpen, and with good reason.

Through the All-Star break, Dusty Baker's beleaguered bullpen was the worst in baseball, pitching to a 5.20 ERA and causing the Nationals to lose six games in which they held a lead after eight innings, the most in the majors and two more times than all of last season. But on July 16, GM Mike Rizzo swung a deal with Oakland that brought Doolittle and fellow A's reliever Ryan Madson to Washington. Two weeks later, Rizzo picked up Twins closer Brandon Kintzler in a trade that just barely beat the July 31 trade deadline buzzer. Since then, the Nats' pen -- which was the glaring weakness for a team that's running away with the NL East -- has become a position of strength.

The Nationals' new late-game vibe is that narrow leads are something that Sean Doolittle and the new-look pen can deliver on. Greg Fiume/Getty Images

On Thursday against Miami, Kintzler entered in relief of starter Tanner Roark with the score tied 2-2 and proceeded to pitch a clean top of the seventh. The righty's fifth straight scoreless appearance since coming over from Minnesota came against the top of the Marlins' lineup and included Kintzler retiring Giancarlo Stanton -- who had homered earlier in the game and has gone deep 18 times in his past 30 games -- on a grounder to second. Next came Madson, who threw a 1-2-3 eighth inning (including two K's) and hasn't been scored on in any of his eight outings as a National. After rookie Brian Goodwin homered in the bottom of the eighth to give Washington a 3-2 lead, Doolittle emerged from the bullpen while "For Whom the Bell Tolls" blared over the Nats Park sound system. The Metallica tune is nothing new for Doolittle, who used it as his walkout song in Oakland. What was new was the way that Nats fans responded to the lefty reliever by belting out his nickname in unison.

"I think that was the first time here," said Doolittle, who had a bit of a rocky start in Washington, giving up four earned runs and four walks over his first four outings. Since then, the former A's setup man has settled quite nicely into his role as capital closer, thank you very much, tossing six shutout innings over his past six appearances, giving up four hits and no walks. Despite the early hiccups, he's now 7-for-7 in save opportunities since the trade and is starting to become a fan favorite in D.C.

"I think they saw the jerseys, the Players Weekend shirts," said Doolittle, referring to the special edition uniforms that players will wear on the last weekend of August and will feature players' nicknames. Although Doolittle's popular Twitter account (@whatwouldDOOdo) highlights his handle, it seems folks in Washington are just now catching on.

"I was very fortunate the way the fans were in Oakland," Doolittle said, "and it was kind of a really similar feeling here tonight. It was really cool. I did hear them a little bit between pitches, between hitters. It made me feel good."

Doolittle's not the only one feeling good.

"Everybody feels a lot more comfortable," said Baker, whose club seems buoyed by the bullpen's beefed-up back end. On July 26 against Milwaukee, the Nats followed a scoreless top of the eighth by Madson with a seven-run bottom half that helped them turn a 2-1 deficit into an 8-5 win. This past weekend against the Cubs, Washington trailed 4-1 after six innings but then exploded for eight runs over the final three frames as Kintzler, Madson and Doolittle combined to throw up three goose eggs. On Thursday, it was more of the same.

"Those guys are coming in and shutting people down," said Goodwin, whose 13th homer of the year helped extend Washington's division lead to 15 games, a season high. "They're going right at hitters and attacking them, and they're being kind of vicious about it. Just the way they're throwing the ball, they're commanding the ball, and their presence on the mound, it's relaxing for us."

Come October, when tensions run high, that kind of calm could come in handy.