The letter comes nearly one week after a top building trades union launched a midterm-election assault against House Democrats who oppose the $5.4 billion oil-sands pipeline.
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Tom Steyer is vowing to throw his political weight and money behind any lawmaker in Congress who comes under attack for opposing the Keystone XL pipeline.

The billionaire hedge fund manager turned environmental activist penned an open letter on Thursday, pledging to utilize his primarily self-funded super-PAC, NextGen Climate Action, to back members of Congress that face attack from pro-Keystone XL groups heading into the midterms.