Shayne Walker, a weld inspector, fills out paperwork during construction of the Gulf Coast Project pipeline, which is part of the Keystone Pipeline project, in Prague, Okla., recently. (Photo by Daniel Acker/Bloomberg)

Poll after poll has shown overwhelming support for building the Keystone XL oil pipeline. So when Congress passes a bill authorizing its construction, people want the White House to sign it immediately, right?

Wrong.

A new poll from the Washington Post and ABC News gets at this question in a different way, and it suggests that support for Keystone is softer -- and less urgent -- than previously thought. The survey asks people whether they think the pipeline should be authorized now, or whether they think a review should be completed to determine that the project is in the nation's interest.

Just 34 percent of Americans say, 'Build it now.' An additional 61 percent are happy to allow the review process to play out.

Even among Republicans, 43 percent want the review to run its course. Just more than half -- 53 percent -- insist it's time to move forward.

That overall 61-34 split is the reverse of most Keystone polls. The most recent Washington Post-ABC News poll showed that Americans supported the project by a margin of 65 percent to 22 percent. A June Pew Research Center poll, meanwhile, showed showed a split of 61 percent to 27 percent in favor of the pipeline, with six of seven ideological groups overwhelmingly in favor -- all but "solid liberals."

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So what's going on here? A few ideas:

1) The idea of completing a review is something many people can get behind. It's a pretty innocuous concept that, after all, could result in the pipeline being built anyway.

But opponents note that the review has been going on for years, and they contend that the Obama administration and the State Department are dragging their feet and will just reject the project anyway.

2) There could be less urgency to build the pipeline now that gas prices are dropping and the economy is recovering. Its supporters play up the jobs it would create and the efficiencies created for refining oil. That was probably a lot more attractive when people were paying $4 per gallon rather than $2 and the recovery was more uneven.

Support for the pipeline hasn't waned -- a CBS News poll released last week showed a similar split of 60 percent to 28 percent in favor of building the pipeline -- but it's possible that it's just not as pressing as it was before.

3) Support for the pipeline is wide but shallow. We doubt that too many people have thought long and hard about this issue, including both the economic and environmental impacts. Thus they are less insistent that it be built immediately.

It's something to remember as the White House faces a likely veto of the Keystone XL bill. As long as they cast it as being out of an abundance of caution, this poll suggests that even many Americans who want the pipeline will understand.

But if and when they ultimately nix the project altogether -- as most think they will -- they will still be running afoul of the vast majority of Americans.