It would be a political embarrassment if they cave. But the policy implications are much less clear. The provision would require the administration to decide within two months whether to greenlight a pipeline that would run from the Canadian tar sands across the U.S. border, through the plains states, to the Gulf of Mexico. Specifically, it would require the State Department to issue the permit for the pipeline, unless President Obama determines it's not in the national interest. It's a make or break issue for environmentalists, who strongly oppose the Keystone pipeline -- for domestic environmental risks, yes, but much more so because they believe there are enough fossil fuels in those tar sands to make climate change irreversible.
The administration wanted to postpone its decision on the pipeline until after the election. And the State Department, which has jurisdiction over the decision, has warned that by forcing the issue, Congress will likely kill it.
Republicans know that, but they want to pick the fight anyway. And if they win, they'll press Obama not to give the pipeline the thumbs down.