"That's why we have to be much smarter in how we deal with Putin and how we deal with his ambitions," Clinton said at a campaign event in Glen, New Hampshire. "He's not an easy man… But I don't think there is any substitute other than constant engagement."
Extolling US foreign policy, Clinton previously imitated the Russian president internal thought process delivering a rebuke to his decision to run for a third non-consecutive term for the Russian presidency in 2012.
Vladimir Putin did not leave Hillary Clinton's remarks unattended noting "Mrs. Clinton has never been known for the elegance of her remarks".
This February she blamed European governments for being "too wimpy" in dealing with Putin.
"Her general anxiety was that Putin, if unchallenged and unchecked, would continue to expand his influence in the perimeter of what was the Soviet Union," the CNN television channel cited London Mayor Boris Johnson as saying.
However, the Democratic Party's frontrunner is putting harsh on the Russian president, she seemed more careful now about her rhetoric to the address of Putin.
The Clinton Foundation has been scrutinized as continuing reports claim that the organization accepted millions of dollars in foreign donations while Clinton served as the US Secretary of State, potentially leading to conflict of interest, which Clinton tried to avoid promising to publicly disclose all donors to the Foundation.
But a review of tax records in Canada shows that he has donated millions of dollars more, along with other contributors with ties to Uranium One or UrAsia, the New York Times reported.
In 2013, a deal was completed on the acquisition of 51 percent of Uranium One by the Russian atomic energy agency Rosatom.