Simple economics tells us why cruise ships can charge $15 for sun block, movie theaters can charge $7 for a Coke and hotels can charge $11 for hair spray — they, and they alone, control access to the product, and you are a captive audience. So, if you’re a pale, thirsty slob, you have to be prepared to whip out your credit card and pay up.

In politics, the same goes for access to politicians. If you want something changed in Washington, D.C., or your state capital, you will most likely have to go through your U.S. senator, congressman or governor to get it done. These politicians control access to the levers of power, and every voter only has two senators, one congressman and one governor.

If one of these politicians were to aspire to higher office, they would count on their home state donors, campaign operatives and voters to propel them to the nomination and ultimately to the White House.

However, if a state has two candidates vying for a presidential nomination, those resources get split up, and the “home state advantage” becomes the “home state scramble,” as was the case in the 2016 Republican primary when former Florida Gov. Jeb Bush and one of Florida’s U.S. senators, Marco Rubio, both threw their Miami Dolphins hats into the presidential ring.

Needless to say, neither candidate ever caught any traction and both ended up getting stomped all over by Donald Trump.

The 2020 presidential race could have similar dynamics at play, at least on the Democratic side of the aisle — as California could have as many as seven different candidates on the ballot.

Potential Golden State candidates in the Democratic primary include San Francisco enviro-sugar daddy Tom Steyer, Lieutenant Gov. and current front-runner in the race for governor Gavin Newsom, U.S. Sen. Kamala Harris, current Gov. Jerry Brown and Los Angeles Mayor Eric Garcetti.

Additionally, Burbank Rep. Adam Schiff and Bay Area Rep. Jackie Speier could attempt to parlay their newfound celebrity from being cable news chatterboxes on the Russia investigation and the #MeToo movement into presidential runs.

Now we know why so many California Democrats support single-payer health care — tons of them have been diagnosed with a nasty case of Potomac fever.

For the candidates, this will be a logistical disaster. However, California Democratic voters will be courted like they’re the executor of Prince’s estate.

Having two candidates from the same party and state is a pill; having an entire field of candidates from the same place turns every theoretical advantage into a liability.

My suspicion is that before the voting starts, major California Democratic Party donors will have to have their own “primary” where they determine who has viability and who doesn’t. Those who don’t make the cut will be frozen out and forced to compete without the open pipeline to campaign cash — political Siberia.

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If Democratic donors don’t do that, the 2020 Democratic primary will be a very long, expensive race that will harm relationships beyond repair and potentially cause fatal damage to the eventual nominee. This reality would give President Donald Trump a strategic advantage as he prepares for re-election.

In the midterm elections, we’ve seen a record number of Democratic candidates file for races up and down the ballot. There’s no indication that the 2020 presidential race will be any different.

If Democratic Party power-brokers can’t winnow the field down before the voting starts, 2020 is going to be a wild ride — and hailing from the richest, most populated state in the union might be an unrecoverable liability.

John Phillips is a CNN political commentator and can be heard weekdays at 3 p.m. on “The Drive Home with Jillian Barberie and John Phillips” on KABC/AM 790.