A whopping 20 percent of Republican primary voters are military veterans. No wonder GOP presidential candidates are making the scandal-plagued Veterans Affairs Department a campaign issue.

And the man frequently tagged as lacking substance — Donald Trump — just came out with the best reform plan so far.

Under Trump’s plan, vets of all ages who are eligible for VA care could get treated at any civilian doctor’s office or hospital that takes Medicare. No bureaucratic hoops, pre-approvals or wait lists.

Trump’s proposal leverages the close connection between Medicare and Veterans Affairs — 45 percent of VA users are already on Medicare. Opponents are carping that Trump’s plan lacks details, but we don’t need a 1,000-page bill. Trump’s proposal works. It also does an end-run around the VA bureaucracy that has sabotaged every effort so far to allow vets to go outside for care.

For 20 years, Veterans Affairs has failed to provide timely care to sick vets. Congress has been alerted again and again that VA schedulers falsified wait lists and even lied to vets, telling them they were in line to be seen when they weren’t. After several vets died needlessly at the Phoenix VA facility, Congress finally enacted reform last year. It promised vets the option of civilian care.

Every vet gets a “choice card,” but it’s a cruel joke. The law erects so many hurdles that few vets can actually use it.

Vets have to live more than 40 miles from a VA facility or have waited more than 30 days to be eligible. Then they have to get a letter confirming eligibility from Veterans Affairs — good luck with that. Then their civilian doctor has to call for pre-approval before treatment — fat chance getting that call answered. After all that, treatment is limited to 60 days. Like you can cure cancer in 60 days!

More than half of vets haven’t even heard about the choice program, which is just what the VA bureaucracy wants. But count on Trump — the master marketer — to get the word out about his plan if he’s elected president. And to fire VA Secretary Robert McDonald, who is a mere apologist for the system.

Competitors criticize Trump for not estimating the cost. But the facts suggest his plan will be low-cost, because nearly half of VA users are already on Medicare. Astoundingly, over 1 million of them have Medicare Advantage, which means taxpayers are paying insurance companies to care for them and then paying Veterans Affairs to care for them. This nonsensical double payment sends 10 percent of the VA health budget down the drain every year.

Overall, encouraging older vets to use Medicare instead by picking up their out-of-pocket costs when they go to a civilian doctor is a no-brainer, and it would cut VA wait lists in half. That would allow younger vets to see a VA doctor faster. But Trump also gives them access to civilian doctors, if they prefer.

Vets needing procedures such as bypass surgery and angioplasty do better at civilian hospitals that perform them in high volume. Medical studies support Trump’s idea. (No kidding!)

Concerned Veterans for America lashed out at Trump’s plan for lacking details. It proposes moving vets into private health plans with subsidies to help pay the premiums. But the plans resemble ObamaCare, meaning most vets could face large out-of-pocket costs.

That’s a nonstarter. Vets already earned their care. At the VA, they can see a doctor for $15. Maybe Concerned Veterans was fast to criticize Trump because one of their plan’s authors is an adviser to another presidential campaign.

Hillary Rodham Clinton says she’ll unveil a VA reform plan next month. She’s already brushed aside evidence of VA corruption, denying it’s widespread. Trump is the candidate best equipped so far with a real alternative when she launches into a defense of the VA’s rotten status quo.

Betsy McCaughey is the author of “Beating ObamaCare” and a senior fellow at the London Center for Policy Research.