Officials at the environmental groups were mum, though they did tip their hands by announcing a 9 a.m. conference call with reporters to “make a major endorsement announcement for the 2012 election.”

“I can tell you that we will not be endorsing Rick Santorum,” Sierra Club Executive Director Michael Brune told POLITICO on Tuesday.

For the Sierra Club, the Obama endorsement will be the earliest ever given in a presidential contest. It backed John Kerry in May 2004 but didn’t get behind President Bill Clinton’s reelection bid until September 1996, a late date due to unhappiness over his signing of a 1995 budget bill including logging language green groups opposed.

The Sierra Club also endorsed Clinton’s initial White House run in September 1992. It made its first presidential endorsement in its nearly 100-year history for Democrat Walter Mondale in September 1984.

LCV endorsed Kerry ahead of the New Hampshire primary in January 2004 — which is still the earliest endorsement since the group was founded in 1969.

In 2008, Obama received the Sierra Club’s endorsement in June and LCV’s in July after his primary fight against Hillary Clinton.

While it’s no surprise greens will back Obama, he still had some work to do to mend fences after failing to pass a comprehensive climate bill and then punting new smog limits.

Obama has thus far rebuked Republican efforts to expedite permitting of the Keystone XL pipeline, issuing repeated veto threats and calling Democratic senators to twist arms ahead of a key vote earlier this year.

On Tuesday, Obama again said he would reject a 90-day House Republican surface transportation law extension that would authorize TransCanada’s proposed 1,700-mile project carrying crude oil from Alberta oil sands to Texas refineries.

The Sierra Club’s Brune said it would be “a shot straight to the gut” if Obama were to fall on his sword and sign any measure authorizing the pipeline project.

“Our members would be furious and would express themselves,” Brune said. “But it’s not something that we expect will happen as the president indicated today.”

Jonathan Allen contributed to this report.

This article first appeared on POLITICO Pro at 7:49 p.m. on April 17, 2012.