You are discounting the aggregate of population size. That 19% you lament is only a small percentage of the whole but a large segement of actual votes.
That 19% in the states like NY, CA, IL, PA, etc. have more total votes that dozens of states combined. They alone can wipe out the entire votes in states even like mine - TN.
If they get enough in Houston, Austin, and Dallas to pad their national total they don’t need to win Texas as a state persay.
And almost all medium to large cities and counties even in red states lean to the Dems. Nashville, Memphis, Little Rock, Birmingham, New Orleans, Phoenix, Tuscon, Las Vegas, St. Louis, etc.
Why campaign accross a red state when all you need is to increase turnout in these cities to, again, pad your national vote total enough to win. No need to win states.
With the current state-by-state winner-take-all system of awarding electoral votes, it could only take winning a bare plurality of popular votes in the 11 most populous states, containing 56% of the population of the United States, for a candidate to win the Presidency — that is, a mere 26% of the nation’s votes.
If big cities controlled the outcome of elections, the governors and U.S. Senators would be Democratic in virtually every state with a significant city.
Even in California state-wide elections, candidates for governor or U.S. Senate don’t campaign just in Los Angeles and San Francisco, and those places don’t control the outcome (otherwise California wouldn’t have recently had Republican governors Reagan, Dukemejian, Wilson, and Schwarzenegger). A vote in rural Alpine county is just an important as a vote in Los Angeles. If Los Angeles cannot control statewide elections in California, it can hardly control a nationwide election.
In fact, Los Angeles, San Francisco, San Jose, and Oakland together cannot control a statewide election in California.
Similarly, Republicans dominate Texas politics without carrying big cities such as Dallas and Houston.
There are numerous other examples of Republicans who won races for governor and U.S. Senator in other states that have big cities (e.g., New York, Illinois, Michigan, Pennsylvania, and Massachusetts) without ever carrying the big cities of their respective states.