Mitt Romney is vowing to approve TransCanada's Keystone XL pipeline on his first day on the job if elected U.S. president in November.

In a campaign ad unveiled today, the presumptive Republican nominee asks voters to imagine Day 1 of his presidency, and lists Keystone as a top priority.

The ad's narrator says approving the Alberta to Texas pipeline would create thousands of jobs that U.S. President Barack Obama has blocked. 

Romney has been maligning Obama for months for rejecting the $7-billion pipeline earlier this year.

But Obama invited TransCanada to apply for another permit once the Calgary-based company rerouted the project around a crucial aquifer in Nebraska.

TransCanada recently came up with a new route that skirts the aquifer before joining up with the original proposed path of the pipeline.

But environmentalists say the new route continues to pose threats to Nebraska's drinking water since it still traverses the aquifer.