(CNN) Bernie Sanders sharply criticized Hillary Clinton for taking campaign contributions from energy lobbyists on Sunday -- but he wouldn't back a high-profile surrogate's criticism of Clinton's email controversy.

The Democratic presidential contenders have traded shots in recent days over contributions from those with fossil fuel ties. Sanders said the problem isn't individual contributions from those working for energy companies -- but Clinton and her super PAC taking money from oil and gas lobbyists.

"Let the voters decide whether paid lobbyists who represent the fossil fuel industry, 43 of them, gave maximum contributions to the Clinton campaign and whether or not these same people are out in some cases are out in some cases bundling, trying to bring in even more money," Sanders told CNN's Jake Tapper on "State of the Union."

He added: "There is a difference between getting money from a worker and somebody whose job it is to represent that industry. I believe we've got to take on the fossil fuel industry."

Clinton and her campaign have accused Sanders of lying about oil and gas contributions, calling him desperate, and have pointed out that he, too, has taken money from those with oil and gas ties.

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