ABC's Dan Lopez reports on Twitter:

BREAKING NEWS: An Obama 2012 staffer, Alex Okrent, collapsed at Obama HQ today & later died at a hospital in Chicago.

— Dan Lopez (@4danlopez)
July 13, 2012

The Chicago Sun-Times confirms:

Alex Okrent, a staffer in the Obama for America campaign who has worked with Barack Obama’s political team since 2004, died Friday after collapsing in the Chicago headquarters. President Barack Obama called his family to offer his condolences. Obama later consoled campaign staffers in a dial-in conference call on Friday afternoon. Okrent worked in the campaign’s paid media department, which handled advertising for Obama’s re-election bid. He was a staffer for Obama’s 2008 presidential campaign and worked on his 2004 race for an Illinois Senate seat. Okrent also worked in a variety of other campaigns.

UPDATE: Mitt Romney sends his condolences:

Ann & I were deeply saddened to learn of the death of Alex Okrent. Prayers are with Alex’s loved ones and the entire Obama campaign team.

— Mitt Romney (@MittRomney)
July 13, 2012

And Axelrod replies:

@MittRomney Thank you, governor. Alex was a beloved member of our team. It's been a very emotional day.

— David Axelrod (@davidaxelrod)
July 13, 2012