| Jean Arthur | | Clarissa Saunders |
| James Stewart | | Jefferson Smith |
| Claude Rains | | Sen. Joseph Harrison Paine |
| Edward Arnold | | Jim Taylor |
| Guy Kibbee | | Gov. Hubert 'Happy' Hopper |
| Thomas Mitchell | | Diz Moore |
| Eugene Pallette | | Chick McGann |
| Beulah Bondi | | Ma Smith |
| H.B. Warner | | Sen. Agnew (Senate Majority Leader |
| Harry Carey | | The Vice President |
| Astrid Allwyn | | Susan Paine |
| Ruth Donnelly | | Emma Hopper |
| Grant Mitchell | | Sen. MacPherson (Privileges and Elections Committee |
| Porter Hall | | Sen. Monroe (objecting to frivolity |
| Pierre Watkin | | Sen. Barnes (Senate Minority Leader |
| Charles Lane | | Nosey newsman |
| William Demarest | | Bill Griffith (at station |
| Dick Elliott | | Carl Cook (at station |
| Billy Watson | | Peter Hopper |
| Delmar Watson | | Jimmie Hopper |
| Johnny Russell | | Otis Hopper (as John Russell |
| Harry Watson | | Hopper boy |
| Garry Watson | | Hopper boy (as Gary Watson |
| Larry Simms | | Hopper boy (as Baby Dumpling |
| H.V. Kaltenborn | | Himself (broadcaster |
| Erville Alderson | | Handwriting expert (uncredited |
| Stanley Andrews | | Sen. Hodges (uncredited |
| William Arnold | | Reporter (uncredited |
| Sam Ash | | Sen. Lancaster (uncredited |
| Edwin August | | Senator (uncredited |
| Frank Austin | | Inventor at Smith's office door (uncredited |
| Harry A. Bailey | | Sen. Hammett (uncredited |
| Wade Boteler | | Family man (uncredited |
| Ed Brewer | | Senate reporter (uncredited |
| Al Bridge | | Sen. Dwight (uncredited |
| Harlan Briggs | | Mr. Edwards (howling citizen |
| Tommy Bupp | | Boy cheering for Smith in meeting (uncredited |
| Frederick Burton | | Sen. Dearhorn (uncredited |
| Ken Carpenter | | Announcer (uncredited |
| Jack Carson | | Sweeney Farrell (newsman |
| Burr Caruth | | Townsend (uncredited |
| Maurice Cass | | Handwriting expert (uncredited |
| Allan Cavan | | Ragner (newsman |
| Eddy Chandler | | Reporter (uncredited |
| George Chandler | | Reporter (uncredited |
| Dora Clement | | Mrs. McGann (uncredited |
| Edmund Cobb | | Sen. Gower (uncredited |
| Dorothy Comingore | | Woman (at station |
| Hal Cooke | | Reporter (uncredited |
| George Cooper | | Waiter (uncredited |
| Jack Cooper | | Photographer (uncredited |
| Nick Copeland | | Senate reporter (uncredited |
| Anne Cornwall | | Senate reporter (uncredited |
| Gino Corrado | | Barber (uncredited |
| Maurice Costello | | Diggs (newsman |
| Alec Craig | | Speaker (uncredited |
| Beatrice Curtis | | Paine's secretary (uncredited |
| Dulcie Day | | Senate reporter (uncredited |
| Vernon Dent | | Senate reporter (uncredited |
| Clyde Dilson | | Reporter (uncredited |
| Neal Dodd | | Senate chaplain (uncredited |
| Ann Doran | | Paine's secretary (uncredited |
| Helen Jerome Eddy | | Paine's secretary (uncredited |
| Jack Egan | | Reporter (uncredited |
| Douglas Evans | | Francis Scott Key (uncredited |
| Eddie Fetherston | | Senate reporter (uncredited |
| Mabel Forrest | | Senate reporter (uncredited |
| Byron Foulger | | Hopper's secretary (uncredited |
| Michael Gale | | Senate reporter (uncredited |
| Jack Gardner | | Reporter (uncredited |
| Frances Gifford | | Hopper girl (uncredited |
| June Gittelson | | Woman (at station |
| Mary Gordon | | Woman (uncredited |
| Lorna Gray | | Woman (at station |
| Harry Hayden | | Speaker (uncredited |
| Louis Jean Heydt | | Soapbox speaker (uncredited |
| Olaf Hytten | | Hopper's butler (uncredited |
| John Ince | | Sen. Fernwick (uncredited |
| Lloyd Ingraham | | Committeeman (uncredited |
| Frank Jaquet | | Sen. Byron (uncredited |
| Dickie Jones | | Pageboy Richard Jones (uncredited |
| Eddie Kane | | Reporter (uncredited |
| Robert Emmett Keane | | Editor (uncredited |
| Donald Kerr | | Reporter (uncredited |
| Milton Kibbee | | Senate reporter (uncredited |
| Joe King | | Summers (newsman, 'He's just decorating a chair' |
| Evalyn Knapp | | Reporter (What do you think of the girls in this town? |
| Wright Kramer | | Sen. Carlton (uncredited |
| Vera Lewis | | Mrs. Edwards (uncredited |
| Arthur Loft | | Chief clerk (uncredited |
| Hank Mann | | Photographer (uncredited |
| Margaret Mann | | Nun with cheering orphan boys (uncredited |
| Philo McCullough | | Sen. Albert (uncredited |
| Matt McHugh | | Reporter (uncredited |
| George McKay | | Reporter (uncredited |
| Lafe McKee | | Civil War veteran at Lincoln Memorial (uncredited |
| James McNamara | | Reporter (uncredited |
| Robert Middlemass | | Speaker (uncredited |
| James Millican | | Senate reporter (uncredited |
| Charles R. Moore | | Porter (uncredited |
| Gene Morgan | | Reporter (uncredited |
| William Newell | | Reporter (uncredited |
| Alex Novinsky | | Foreign diplomat (uncredited |
| Frank O'Connor | | Sen. Alfred (uncredited |
| Frank Puglia | | Handwriting expert (uncredited |
| Ed Randolph | | Senate reporter (uncredited |
| Jack Richardson | | Sen. Manchester (uncredited |
| Russell Simpson | | Kenneth Allen (uncredited |
| Walter Soderling | | Sen. Pickett (uncredited |
| Harry Stafford | | Sen. Atwater (uncredited |
| Wyndham Standing | | Sen. Ashman (uncredited |
| Paul Stanton | | Flood (newsman |
| Count Stefenelli | | Foreign diplomat (uncredited |
| Craig Stevens | | Senate reporter (uncredited |
| Carl Stockdale | | Sen. Burdette (uncredited |
| Dub Taylor | | Reporter (uncredited |
| Ferris Taylor | | Sen. Carlisle (uncredited |
| Arthur Thalasso | | Doorman (uncredited |
| Fred 'Snowflake' Toones | | Porter (uncredited |
| Victor Travers | | Sen. Grainger (uncredited |
| Laura Treadwell | | Mrs. Taylor (uncredited |
| John Tyrrell | | (uncredited |
| Robert Walker | | Sen. Holland (uncredited |
| Myonne Walsh | | Jane Hopper (uncredited |
| Lloyd Whitlock | | Schultz (newsman |
| Dave Willock | | Senate Guard (uncredited |
| Florence Wix | | Committeewoman (uncredited |