Update

In just under 7 hours, the 500,000 signature goal was reached. No one was sure if it could be reached in 24 hours. Bill McKibben tweeted: “Um, I don’t quite believe it. We just hit 500 k emails in 6:54. Whaddya say we just keep going?”

Over the last year, environmental groups have relied on old fashioned boots-on-the-ground protests to halt the Keystone XL tar sands pipeline. Now they’re turning to the web for a last-minute attempt to prevent Congress from resurrecting the project.

Starting today at noon, a coalition of environmental organizations including 350.org, Sierra Club, Natural Resources Defense Council, and CREDO Action, have rolled out a “signature bomb” in an attempt to get 500,000 citizens to tell Senators to stop trying to force a decision on Keystone XL.

No environmental organization has ever been able to raise that many signatures so quickly. But groups leading the effort are hoping they can use the momentum from last year’s protests to drum up the needed support.

“The environmental community is coming together in a way that it hasn’t in a long time,” said 350.org Founder Bill McKibben in a conference call today. “We’ve never tried anything like this before. But the environmental movement is well wired and well connected, so we’re going to see how this plays out.”

Keystone XL is a proposed 1,700 mile pipeline that would bring energy and water-intensive crude from Canada’s tar sands to refineries in Texas for sale in the global market. Sometimes called the “scar sands,” Alberta’s tar sands have been dubbed the “the most destructive project on earth” due to the immense amount of water, land, and fossil fuels needed to strip oil out of bituminous sand.

Last summer, most “insiders” in Washington believed the project would be easily approved. But after a series of successful demonstrations in front of the White House — combined with an arbitrary deadline for approval placed on the White House by Congress at the end of the year — the president rejected the permit last month.

But Congress won’t let it die. House Majority Leader John Boehner continues to tell reporters that supporters will “do everything we can” to make sure Keystone is approved. One way is to add an amendment in the transportation bill currently being considered in the House and Senate that would allow Congress to approve the pipeline.

That’s exactly what Republican leaders in the Senate plan to do this week. Senators Dick Lugar (R-IN), John Hoeven (R-ND), and David Vitter (R-LA) may add an amendment any day now that gives Congressional lawmakers authority to circumvent executive authority and green-light the controversial pipeline. Any legislation would still need to be signed by President Obama, however.

“This is the zombie pipeline that keeps coming back to life,” said McKibbnen. “We need the Senate to back up the President. These emails will be flooding into the Senate over the next 24 hours and hopefully they’ll make an impression.”

The environmental community already made an impression far bigger than expected a year ago. Will it be able to maintain its strength?