Environmental groups have raised new concerns about the close ties between Barack Obama, Hillary Clinton and lobbyists for the controversial Keystone XL project to carry crude from the tar sands of Alberta to the refineries of Texas.

Friends of the Earth said on Thursday they were expanding their freedom of information request for emails between State Department officials and the pipeline company after learning of seven more pipeline lobbyists who had worked on Obama and Clinton's presidential campaigns, or had ties to Bill Clinton.

The move comes a day before what is expected to be a contentious last hearing of public concerns about the pipeline before the State Department renders a decision on whether to give the project the go-ahead.

Environmental groups trying to block the pipeline had initially focused on the role of Paul Elliott. Elliott, the main lobbyist in Washington for TransCanada pipeline company, had been a senior official on Clinton's campaign in 2008.

DeSmogBlog, a website which specialises in tracking corporate influence on environmental policy said it had identified seven other lobbyists with close ties to Obama and Clinton.

"Lobbyists with strong ties to Secretary Clinton are spread out over three firms, including one that was the largest single source of funds of any corporate entity to Clinton's 2008 presidential run," Desmogblog said.

The most prominent of these is Jeff Berman, now of Bryan Cave LLC, who was described at the time as "Obama's secret weapon" in his contest against Clinton.

Berman's mastery of the delegate process was crucial to securing Obama's victory over Clinton in a long and gruelling primary battle. In some states, Obama managed to secure more delegates – thus coming closer to the nomination – than Clinton, even though she had won more of the popular vote.

DeSmogblog argued those close connections between political aides and industry undermined Obama's very public promises during the 2008 campaign to sever the ties between lobbying and government.

"The presence of so many former Clinton associates on the lobbying roster for polluter clients on a high-profile controversy suggests a clash with the repeated campaign pledges of greater transparency and tougher dealings with lobbyists by Secretary Clinton's boss, President Obama," the blog said.