The White House on Thursday waded into the escalating duel between Hillary Clinton and Bernie Sanders over who is qualified to be president, and flatly rejected Sanders' charge that Clinton doesn't have what it takes to be the leader of the free world.

Presidential spokesman Eric Schultz said Clinton "comes to the race with more experience that any non-vice president" in recent campaign history.

Schultz also noted that Obama was "fortunate" to have her serve in his administration and is "proud of that service."

President Obama has tried to stay neutral in the Democratic primary, which has become more competitive than expected. But Thursday's comments backing Clinton's credentials and shooting down Sanders' criticism of her are some of the most directly favorable to Clinton's candidacy so far.

On Wednesday, the White House said that Obama will continue to refrain from taking sides in the primary, but wouldn't hesitate to jump in "with zest" in the general election to advocate for a Democratic successor.

The race for the Democratic presidential nomination took a sharply negative turn Wednesday when each candidate questioned the other's fitness to lead the nation.

At a rally in Philadelphia, Sanders took issue with Clinton's recent criticism of his New York Daily News interview, which she said showed a lack of understanding of financial regulation policy.

After weeks of more subtle jabs, Sanders took the gloves off and assailed Clinton, impugning her qualifications after her super PAC took millions from Wall Street, noting that she voted in favor of the Iraq war and several trade deals he argued have cost American workers million of "decent paying jobs."