Python 2, 329 bytes

print'Lee Burr Cruz Enzi King Paul Reed Blunt Brown Casey Coons Crapo Ernst Flake Hatch Kaine Leahy Moran Risch Rubio Sasse Scott Thune Udall Wyden Young Bennet Booker Capito Cardin Carper Corker Cornyn Cotton Daines Durbin Graham Harris Hassan Heller Hirono Hoeven Inhofe Markey McCain Murphy Murray Nelson Perdue Peters Rounds'

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String Generated by this program(get list from the data string, sort the strings by length, take first 51 elements)

Python 3, 914 bytes

a='Alexander, Baldwin, Barrasso, Bennet, Blumenthal, Blunt, Booker, Boozman, Brown, Burr, Cantwell, Capito, Cardin, Carper, Casey, Cassidy, Cochran, Collins, Coons, Corker, Cornyn, Cortez Masto, Cotton, Crapo, Cruz, Daines, Donnelly, Duckworth, Durbin, Enzi, Ernst, Feinstein, Fischer, Flake, Franken, Gardner, Gillibrand, Graham, Grassley, Harris, Hassan, Hatch, Heinrich, Heitkamp, Heller, Hirono, Hoeven, Inhofe, Isakson, Johnson, Kaine, Kennedy, King, Klobuchar, Lankford, Leahy, Lee, Manchin, Markey, McCain, McCaskill, McConnell, Menendez, Merkley, Moran, Murkowski, Murphy, Murray, Nelson, Paul, Perdue, Peters, Portman, Reed, Risch, Roberts, Rounds, Rubio, Sanders, Sasse, Schatz, Schumer, Scott, Shaheen, Shelby, Stabenow, Strange, Sullivan, Tester, Thune, Tillis, Toomey, Udall, Van Hollen, Warner, Warren, Whitehouse, Wicker, Wyden, Young'.split(', ')
a.sort(key=lambda x:len(x))
print(' '.join(a[:51]))

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Python 2, 329 279 bytes

-50 bytes Thanks @caird coinheringaahing: [avoid the delimiter-spaces]

print'LeeBurrCruzEnziKingPaulReedBluntBrownCaseyCoonsCrapoErnstFlakeHatchKaineLeahyMoranRischRubioSasseScottThuneUdallWydenYoungBennetBookerCapitoCardinCarperCorkerCornynCottonDainesDurbinGrahamHarrisHassanHellerHironoHoevenInhofeMarkeyMcCainMurphyMurrayNelsonPerduePetersRounds'

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String Generated by this program(get list from the data string, sort the strings by length, take first 51 elements, join)

a='Alexander, Baldwin, Barrasso, Bennet, Blumenthal, Blunt, Booker, Boozman, Brown, Burr, Cantwell, Capito, Cardin, Carper, Casey, Cassidy, Cochran, Collins, Coons, Corker, Cornyn, Cortez Masto, Cotton, Crapo, Cruz, Daines, Donnelly, Duckworth, Durbin, Enzi, Ernst, Feinstein, Fischer, Flake, Franken, Gardner, Gillibrand, Graham, Grassley, Harris, Hassan, Hatch, Heinrich, Heitkamp, Heller, Hirono, Hoeven, Inhofe, Isakson, Johnson, Kaine, Kennedy, King, Klobuchar, Lankford, Leahy, Lee, Manchin, Markey, McCain, McCaskill, McConnell, Menendez, Merkley, Moran, Murkowski, Murphy, Murray, Nelson, Paul, Perdue, Peters, Portman, Reed, Risch, Roberts, Rounds, Rubio, Sanders, Sasse, Schatz, Schumer, Scott, Shaheen, Shelby, Stabenow, Strange, Sullivan, Tester, Thune, Tillis, Toomey, Udall, Van Hollen, Warner, Warren, Whitehouse, Wicker, Wyden, Young'.split(', ')
a.sort(key=lambda x:len(x))
print(''.join(a[:51]))

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