Decorah, Iowa (CNN)Pete Buttigieg on Thursday accused Joe Biden and Bernie Sanders, by name, of forcing the debate among Democratic presidential contenders to be too focused on the past rather than the future.

Buttigieg's naming of his rivals is a significant shift just days ahead of the Iowa caucuses, something the former mayor of South Bend, Indiana, has avoided doing in recent weeks. The move signals that Buttigieg sees the former vice president and the Vermont senator as his main opponents in the key early state.

"I hear Vice President Biden, saying that this is no time to take a risk on someone new," Buttigieg said. "But history has shown us that the biggest risk we could take with a very important election coming up is to look to the same Washington playbook and recycle the same arguments and expect that to work against a president like Donald Trump, who is new in kind." Earlier this month the Biden campaign released an ad in which a narrator tells voters, "This is no time to take a risk."

Buttigieg then went after Sanders.

"Then I hear Senator Sanders calling for a kind of politics that says you got to go all the way here and nothing else counts," he said. "And it's coming at the very moment when we actually have a historic majority, not just aligned around what it is we're against, but agreeing on what it is we're for."