WASHINGTON — Democratic presidential candidate Barack Obama says he can't imagine saying about an opponent some of the things that his Republican rival, John McCain, has been saying about him.

In an interview aired Thursday on CBS' "The Early Show," Obama points to recent charges on the campaign trail that he is "socialistic" and that he "pals around with terrorists."

Obama says that the campaign for the White House has been tough — but he adds that he believes there is no equivalence between what his campaign has been doing and what the McCain campaign has been doing.

For his part, McCain has said that his TV ads and automated phone calls about Obama's tax policies and his past relationship with a '60s-era radical have been completely accurate.