The trio of rookies sent the sliding Rangers to their eighth straight loss and helped the Mariners to their first series win and sweep of the season. Seattle has won five of six at home with three straight wins against the Rangers coming from 24-year-old rookie starters.
''I think that's the biggest thing,'' Dunn said. ''We're getting comfortable, we're finding our routines, we're getting in a groove and we're having fun - so that's first and foremost.''
Haggerty, an offseason waiver pickup from the New York Mets who's been playing in a utility role due to injuries, made it 4-0 with a two-run home run to left field in the fifth inning. It was his first career home run.
Lewis hit his seventh home run of the season in the first inning, tying White Sox slugger Luis Robert for the major league lead among rookies.
''I say it all the time: That's `Showtime,''' Dunn said of the nickname he's given the AL Rookie of the Year candidate. ''That's all I got for you. Hopefully, in a couple of months here it will be `ROY.' He's a blessed player.''
Austin Nola made it 2-0 in the second with his own solo homer.
The three homers nearly equaled starting pitcher Mike Minor's season total of four home runs allowed entering the game. Minor (0-5) lost his fifth decision in six starts, though he did reach six innings for the first time, allowing five hits while striking out four and walking one.
Minor said he was ''kind of making progress. It was still a bad start.''
Dunn (2-1), one of three 24 year olds in the Mariners' rotation, picked up his second career win - both against the Rangers. He faced one batter over the minimum through four innings, striking out six and walking one in six innings total. His only blemish was Isiah Kiner-Falefa's fifth-inning double.
''I thought his fastball the first couple innings had a lot more life to it today than we've seen recently,'' Mariners manager Scott Servais said. ''Everybody knows he's got a really good breaking ball, but you've got to have something else to go along with it, and today he really did.''
Taylor Williams earned his sixth save.
Texas' losing streak is the longest in the major leagues this year. The Rangers are an AL-worst 3-11 away from home and were swept on a road trip of five-plus games for the first time since June 1991. They've given up up 65 runs during the current slide.
''It is one of those times, honestly,'' Rangers manager Chris Woodward said. ''I don't know if we need to do something drastic like sacrifice a chicken or something, but it's just not going our way.''
With their next 11 games against contenders, Woodward said the Rangers are considering a number of changes. The first could come with the starting rotation due to the struggles of right-hander Jordan Lyles (1-3) and lefty Kolby Allard (0-2), who have both lost two straight.
Lyles has given up 15 earned runs in his last two outings and Allard was chased by the Mariners in the first inning on Friday.
Woodward said openers could be an option. The team also will be looking at bringing in prospects at other positions if veterans continue to struggle.
Rangers: C Robinson Chirinos (ankle) will be activated when Texas returns to Arlington. Woodward said the team may keep three catchers due to the hot play of Jose Trevino and Jeff Mathis.
Rangers: Ace RHP Lance Lynn (3-0, 1.37) takes the mound as Texas returns home to face Oakland on Monday. Lynn is the only Rangers starter above .500 and is averaging a major-league high of 107.8 pitches per game.
Mariners: LHP Marco Gonzales (2-2, 3.34) takes the mound Tuesday as Seattle travels to San Diego on Tuesday after a day off.
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