Actually (like money), yes, there is an endless supply of jobs.
Despite Gator's misunderstanding of the concept, raising the population does increase demand (which increases jobs), but as stated before, you run into problems. 1) You can satisfy the incresaed Demand without employing the entire increased population. 2)The skills of the immigrants may not match the jobs required.
But most of all, it's easier to reap the benefits of the larger market simply by opening up trade rather than dealing with the hassle of immigration. With the consequence that they won't be able to vote Democrat -which is probably the real goal.