Sources: Thurman, Porter headed to welterweight showdown on Dec. 12

Welterweight titlist Keith Thurman and former titleholder Shawn Porter have been on a collision course for a while, and their potential showdown has been one fans have talked about. It appears as though it will happen -- and sooner rather than later.

They are penciled in to fight Dec. 12 (at a site to be determined) on Showtime, multiple sources with knowledge of the fight told ESPN.com. Showtime declined comment on Wednesday.

It would be a marquee fight in one of boxing's best divisions and one that Al Haymon, manager of both fighters and creator of Premier Boxing Champions, finally decided to make.

Thurman (26-0, 22 KOs) would be defending his title for the third time since being elevated to a full titleholder. He made his first defense March 7 in Las Vegas on NBC in prime time in the first-ever PBC main event as he dominated Robert Guerrero in an action-packed but lopsided decision win.

Thurman, 26, of Clearwater, Florida, returned July 11 before a hometown crowd in Tampa for a seventh-round knockout of former titlist Luis Collazo in the first PBC on ESPN card.

Porter (26-1-1, 16 KOs), 27, lost his belt by decision to Kell Brook in August 2014 but rebounded with two impressive performances.

Porter, of Akron, Ohio and living in Las Vegas, scored a fifth-round knockout of Erick Bone on March 13 on the first PBC on Spike TV card in Ontario, California, and followed with a clear decision win against Adrien Broner on June 20 in a prime-time PBC on NBC main event.