If they are so cocksure it won't matter then why not go ahead and do it?
Actually it is worse than you state. The environmentalists (Democrats essentially) want growth to stop. Simply put they want population growth to stop, economic growth to stop, production increases to stop and since they only want this in the USA, it can be concluded that they want other powers to come to be the dominate powers in the future.
Of course you are right that there will be immediate consequences to a decision to drill where there is more oil to be found. (1) unemployment will drop farther as more good paying jobs are “discovered”. (2) the stock market will stop dropping as soon as new profits are factored in because of (3) more hard goods will go into production from oil drilling floating platforms to drilling machinery. (4) fairly soon speculation in oil futures will factor in additional supply. (Some wells will be in production within a year of ending the courtroom attempts to stop drilling that are certain as soon as Congress decides to drill.
BTW, you can see democrats are starting to see that the above represents a political force and they may not want to be on the wrong side.
As a simple analogy, if water is running through a 100' hose attached to a spigot that's open to 10% of maximum flow, how quickly can the flow rate at the output be increased? If the spigot is opened wide, it may take quite a few seconds for any molecules of the "extra" water to make it to the end of the hose, but the flow rate at the output will increase almost instantly.