US presidential hopeful Hillary Clinton has described the Confederate flag as a symbol of America's "racist past" which should not be flown anywhere.
The Democratic candidate's comments come as politicians in South Carolina seek to remove the flag from the grounds of their statehouse.
Major retailers have also halted sales of the flag and related merchandise following the killing of nine black church members in the southern state.
The alleged shooter in the attack, Dylann Roof (21) had posed for photographs with the secessionist banner.
Mrs Clinton described the shootings as an "act of racist terrorism perpetrated in a house of God", and weighed into the debate over the use of the Confederate symbol.
She said the flag has "no place in our present or our future", adding that it should not be flown from the statehouse or anywhere else.
South Carolina's legislature has voted by 103-10 to allow a special session to discuss removing the Confederate banner from the statehouse grounds.
Hundreds rallied on Tuesday outside the chambers in the city of Columbia against the emblem, which was originally flown by the pro-slavery South during the 1861-65 American Civil War.
Online retailer eBay has said it will no longer sell the rebel banner through its website, as did Sears and Etsy.
"Everybody has a right to be represented"
Amazon.com followed suit after purchases of the flag rocketed by more than 3,600% in just 24 hours.
Walmart led the way on Monday saying it would remove all Confederate-themed items from its shelves and website.
On Monday, South Carolina Governor Nikki Haley and other top Republicans called for the flag to be taken down.
But some companies that make Confederate flags have defended the product.
Kerry McCoy, owner and president of Arkansas' FlagandBanner.com, told the Associated Press: "This is America. Everybody has a right to be represented whether you are a history buff or a nut."
Reluctant to alienate white conservative voters, Republican presidential hopefuls have hesitated to echo calls to ban the red-and-white banner.
Momentum has been building for a ban on flying the flag in other US states.
On Tuesday, Virginia's Democratic Governor Terry McAuliffe moved to have the banner banished from state car registration plates.
Mississippi Republican House Speaker Philip Gunn has said the Confederate emblem should be removed from the state flag.
Kentucky's Republican nominee for governor, Matt Bevin, said the state should remove a statue of Confederate President Jefferson Davis from the capitol rotunda.
Calls have also been made to ban Confederate symbols in Alabama, Georgia, Tennessee and Texas.