It’s simple math. A successful political coalition is built on the principle of addition, not subtraction. Basically, you welcome anyone who wants to help. You park all disagreements and join forces for the common cause.

All Democrats should be thrilled that vocal cadres of anti-Trump Republicans are laboring to defeat - by the most decisive margin possible - the worst excuse for a human to ever squat in the Oval Office. But alas, that does not please the Democrats’ purity police. The GOP veterans who are currently agitating against Trump have worked with conservatives in the past, therefore, their sins apparently render them unqualified to help in the present.

Case in point: We got word last week that former Ohio Republican Gov. John Kasich will endorse Joe Biden and address Democrats at the virtual national convention in August. Lots of liberal litmus-testers are predictably horrified. As one of them wrote the other day, “I don’t think that having a ‘big tent’ means it’s necessary to highlight people who would knock down your tentpoles.” And lefty Twitter predictably weighed in: “Kasich is pro-capitalist. The more Dems lean right, the more we lose.”

One big rap against Kasich is that he signed anti-abortion bills and withheld state funds from Planned Parenthood. But his Republican track record is precisely why he should speak for Biden at his convention.

During this unprecedented national emergency, it should be all hands on deck. If Kasich can help persuade even a small percentage of Republicans to switch sides in November - especially in Ohio, which is unexpectedly competitive for Biden - it could have an outsized impact on the results. Trump’s support is eroding - his approval rating on the pandemic has dropped to a record low 32 percent - and Kasich’s old-school GOP credentials make him the perfect person to woo more Republicans.

Is it possible that the purity police have never heard of the old axiom that politics makes strange bedfellows? Isn’t it smart politics for Biden to accommodate the Bernie Sanders wing on the left (which he’s doing) while also reaching out to anyone on the right who cares about the national interest?

Indeed, the progressives who are hostile to Kasich have somehow overlooked the issues they have in common - most notably, Kasich’s gubernatorial decision in 2013 to defy the GOP by expanding Medicaid under Obamacare. When Republicans assailed him for that blasphemy (because any support for Barack Obama was a blasphemy), he said that he wanted to make “real improvement in people’s lives.” He also said: “When you die and get to the meeting with Saint Peter, he’s probably not going to ask you much about what you did about keeping government small. But he is going to ask you what you did for the poor. You better have a good answer.”

Kasich, like Biden, speaks the language of human decency. In this dire election year, there’s a strong yearning for decency, ideally powered by people who can reach across the divide. If Kasich and other anti-Trump Republicans want to help steer the careening American ship, why should it matter what they did five, 10, or 20 years ago?

The liberal purity police would be wise to remember what Ronald Reagan - winner of two decisive presidential elections - once said about strange bedfellows: He wasn’t endorsing them; they were endorsing him.

Uh oh. Is it OK if I invoke Ronald Reagan?

Dick Polman, a veteran national political columnist based in Philadelphia. Email him at dickpolman7@gmail.com.