Bernie Sanders Demands Apology From Hillary Clinton for Accusing Him of Lying The back and forth between Sanders and Clinton has reached a fever pitch.

 -- The back and forth between Bernie Sanders and Hillary Clinton regarding campaign contributions from the oil and gas industry has reached a fever pitch.

Speaking to a packed house eastern Wisconsin, Sanders twice said Clinton owed his campaign an apology.

“We were not lying, we were telling the truth,” the presidential hopeful said after bringing up an incident yesterday in which an activist asked Clinton if she would stop taking money from the fossil fuel industry. Clinton responded aggressively and accused the Sanders' campaign of lying.

“The truth is that Secretary Clinton has relied heavily on funds from lobbyists working for the oil, gas and coal industry, according to an analysis done by Greenpeace,” the senator continued this evening.

A report from the environmental organization found 57 registered K-street oil, gas and coal industry lobbyists contributed to Clinton's campaign and claims over 43 of them contributed the maximum dollar amount allowed for the primary. The report also found several large contributors related to the industry had given more than $3.5 million to one of Clinton’s super PACs.

But the Clinton campaign has not given an inch either. Clinton's National Communication Director Jen Palmieri said today they will not apologize. "Despite their pledge to run a positive campaign about the issues, we have seen increasingly ratcheted-up personal attacks from Senator Sanders, his campaign and their surrogates," she said in a statement. "It’s disappointing that they’ve doubled down on negative character attacks that deliberately mislead voters rather than debate the issues.

"Their latest attack over campaign contributions ignores their own donations from individuals who work for oil and gas companies, and most importantly it ignores Hillary Clinton’s record and agenda. We will not apologize for calling out these kinds of schemes for what they are -- a desperate move from a campaign that has clearly decided that the only hope for a path to victory is through misleading attacks."

Yesterday, the Clinton campaign pointed to additional documents, which show that employees from oil and gas companies have given to both campaigns. According to the Center for Responsive Politics, employees of oil and gas companies have contributed $307,000 to Clinton’s campaign, but also have given $54,000 to Sanders’ campaign.

Sanders seemed to respond to this as he pointed out in his remarks that the contributors with ties to oil and gas cited in the Greenpeace report were “not just workers in the fossil fuel industry, these are paid registered lobbyists.”

Since first announcing his candidacy, the Vermont senator has cited environmental and energy issues, including pipelines, drilling and fracking, as major points of policy disagreement between himself and Clinton. His campaign today sent several releases citing Clinton’s previous campaign contributions from fossil fuels as a senator (the Center for Responsive Politics said in 2008 that she received more donations from the industry than any other Democratic senator) as well as her decision to back certain pipelines -- and not disavow the Keystone XL Pipeline -- when she was secretary of state.

Clinton has since said she is opposed to the pipeline. Her campaign spokesperson Nick Merrill said in a statement Thursday, “Hillary Clinton has a proven record of leadership when it comes to combating climate change and has fought against fossil fuel interests for decades. On this campaign, she has laid out tangible, ambitious goals to make America the clean energy superpower of the 21st century and has repeatedly called for eliminating tax breaks for oil and gas companies.”

ABC News' Liz Kreutz contributed to this report.