As former secretary of state Hillary Clinton made her 2016 presidential campaign official Sunday, Sen. Bernie Sanders (I-Vt.) praised her as "an experienced and well-qualified leader" with whom he was pleased to serve in the Senate. But he also warned her and other potential candidates to "address the great challenges of our time."

Those include economic inequality, unemployment, climate change and campaign finance, he said.

"I hope that Secretary Clinton will speak out on these and other important issues in the days and weeks ahead," Sanders said in a written statement released after Clinton formally entered the race.

The Vermont independent, who has been mulling a presidential campaign of his own, still hasn't decided whether to run, according to spokesman Michael Briggs.

"He is trying to ascertain whether or not there is the grassroots support — in terms of a national volunteer base and small-donor campaign contributors — to mount a successful campaign which takes on the billionaire class and their powerful corporate lobbyists," Briggs said via email. "That decision will be made within a few weeks, certainly by the end of the month."

Even as he's traveled repeatedly to Iowa, New Hampshire and other early presidential primary and caucus states, Sanders has delayed making a final decision. Throughout much of last year, he said he would not make up his mind until after the November midterm elections. In December 2014, he told the Associated Press he'd decide by March 2015.