THE LAST, BEST CHANCE TO OVERHAUL U.S. ENERGY POLICY…. Most objective observers, and more than a few biased ones, will concede that the odds of passing a comprehensive climate/energy bill this year are pretty bad. And given the likely results of the midterm elections, coming up short this year will make meaningful efforts nearly impossible for the next several years.

With that in mind, President Obama has an important task ahead of him tonight, when he addresses the nation from the Oval Office. In addition to updating Americans on the response to the BP oil spill disaster, explaining how BP will be held accountable, and assuring the public that this crisis will not be repeated, the president also has to make the case for a new U.S. energy policy — that the Senate doesn’t want to pass.

The speech, then, may be the last chance to generate support for one of the year’s most important legislative initiatives.

No one outside the White House knows exactly what Obama will say — I doubt the remarks have even been written yet — but officials have told reporters that the president will address “what our fundamental energy approach must be going forward to reduce our dependence on oil and fossil fuels.”

Another official told Marc Ambinder that Obama will use the time to “go Big. That’s where he does best.”

The good news for the White House is that an ambitious message will likely fall on fertile ground. A new national poll from the Pew Research Center shows surprisingly strong support for key policy efforts: 66% of Americans support placing limits on greenhouse gas emissions; 78% support tougher efficiency standards; and 87% favor requiring utilities to produce more energy from renewable sources. Asked if it’s more important to protect the environment or keep energy prices low, the environment wins, 56% to 37%.

So, with all of this in mind, what might a “go big” message look like? Greg Sargent has a worthwhile rhetorical suggestion: “If not now, when? If not us, who?

[I]t’s a reference to the political circumstances created by the Gulf spill. While the health care crisis was and is severe, and touches many lives, the Gulf spill is a slow motion disaster that is dramatizing the consequences of previous inaction with a nonstop gusher of disturbing imagery.

If the Gulf crisis isn’t enough to prompt action by Congress, what would be enough?

By repeating the mantra of “if not now, when” during the health debate, Obama positioned himself as a kind of historical scold, urging members of Congress to rise above petty and parochial political concerns in order to be part of something that would earn them a place in the legislative history books. I don’t know whether it would be enough to move individual Senators, but the argument on its face would arguably be more historically compelling when applied to climate change.

I’d just add that while Obama obviously needs to influence individual senators, he also has to change the larger existing political dynamic. After all, reluctant lawmakers are prepared to let the bill die because they see it as the easier, more politically expedient path.

If the public response to the BP oil spill disaster is a clamoring for a comprehensive energy package, the legislative circumstances can change quickly. Here’s hoping the White House shapes tonight’s message accordingly.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.