Wednesday on TV: Whether GOP or Democrat, "The President's Gatekeepers" is fascinating stuff

The President's Gatekeepers (10 p.m., Discovery) is a truly fascinating two-part, four-hour documentary about the job of White House chief of staff.

All 20 living ex-chiefs of staff are interviewed - Howard Baker, Jr., James A. Baker III, Joshua Bolten, Erskine Bowles, Andrew Card, Dick Cheney, William M. Daley, Kenneth Duberstein, Rahm Emanuel, Jim Jones, Jacob Lew, Denis McDonough, Thomas Mack McLarty, Leon Panetta, John Podesta, Donald Rumsfeld, Samuel K. Skinner, John H. Sununu, Jack Watson and Marvin Watson - along with former presidents Jimmy Carter and George H.W. Bush.

"The White House chief of saff's job has more authority and more power than the vice president," says Chenyey' who's been both.

"Gatekeepers" concludes Thursday.

Elsewhere on TV ...

Big Brother (7 p.m., CBS/Ch. 2): The veto competition.

Million Second Quiz (7 p.m., NBC/Ch. 5): Night 3 of this five-night event.

The X Factor (7 p.m., Fox/Ch. 13): Another season of this much-hyped, not-that-much-watched singing competition begins. Returning judges Simon Cowell and Demi Lovato are joined by Kelly Rowland and Paulina Rubio.

MasterChef (8 p.m., Fox/Ch. 13): A winner is declared in the season finale.

America's Got Talent (8 p.m., NBC/Ch. 5): Six acts advance to the next round.

Nova (8 p.m., PBS/Ch. 7): A look at the construction of One World Trade Center, the 1776-foot tower on the site of the World Trade Towers.

Broadchurch (8 and 11 p.m., BBC America): A new piece of evidence comes to light.

It's Always Sunny in Philadelphia (8 and 9:30 p.m., FXX): The gang engages in the gun ownership debate.

Camp (9 p.m., NBC/Ch. 5): Everyone prepares for the Inter-Camp Olympics.

Brains on Trial with Alan Alda (9 p.m., PBS/Ch. 7): Neuroscience and its potential effect on criminal law.

Royal Pains (10 p.m., USA): Divya receives news about her pregnancy. (Season finale)

Top Chef Masters (11 p.m., Bravo): The chefs create Mexican dishes for wrestlers.

The Bridge (11 p.m., FX): Sonya and Marco close in on the killer.

Duck Dynasty (11 p.m., Discovery): Termites invade Duck Commander.