A day after senior Obama adviser David Plouffe denounced Mitt Romney as a man with “no core,” two other Obama aides added their own stinging criticisms against the Republican presidential hopeful - the latest sign the president’s advisors have zeroed in on Romney as the likely GOP nominee.

As anyone who's watched [Mitt Romney], it’s always a question as to where he was and where he is and where he might be on any given issue,” White House Press Secretary Jay Carney told reporters in an off camera briefing Monday morning. Carney has previously held back direct attacks against Republican presidential contenders.

Carney’s comments came a day after Plouffe, Obama's campaign manager in 2008 and now a senior adviser in the White House, told NBC's "Meet the Press" that Romney's national poll numbers, which are consistently in the low 20% range, showed that most Republican voters were looking elsewhere for a candidate.

“You get the sense with Mitt Romney that, you know, if he thought it was good to say the sky was green and the grass was blue to win an election, he'd say it," Plouffe said.

Romney's campaign said Plouffe's comments were an attempt to distract from what it called "the historic loss of middle-class jobs that has occurred" during Obama's presidency. But Obama campaign spokesman Ben LaBolt called that a “false attack.”

“While the President is fighting for an economy that’s built to last where we outeducate and outinnovate the rest of the world, Mitt Romney has been busy telling Americans what he believes our nation can’t do,” LaBolt said.

With the general election now a year away, the early back-in-forth just might be a first glimpse of the sharp elbows from both sides in the months to come.

UPDATE: The gloves are still off - Andrea Saul, a spokeswoman for Romney's campaign, issued a statement Monday afternoon saying the president's advisors are going to run "a campaign of personal destruction to ‘kill’ Mitt Romney because they are intimidated by his candidacy."

"President Obama knows he can’t run on the failed policies that have brought higher taxes, exploding deficits, and record unemployment, so his allies have no choice but to launch false and personal attacks. Americans know that Mitt Romney is a fiscally responsible businessman who will put in place pro-growth policies that will create jobs and undo the damage caused by the failed Obama presidency," Saul also said in the statement.

- CNN's Jessica Yellin, Alex Mooney, and Tom Cohen contributed to this report