You’re absolutely right. The Democrats could focus entirely on major metropolitan areas, like Chicago and New York, and ignore states like Oklahoma (and even Texas beyond Dallas). This would also make it much easier for them to win elections by voting fraud.
In 2012 Obama beat Romney in CA by 3 million votes and in NY by 2 million votes. So Obama won nationally by 5 million votes and he had a cushion of 5 million from just CA and NY.
In contrast, Romney won Texas by 1.2 million votes.