FILE - In this Jan. 29, 2014, file photo, Larry Hogan, a Republican Maryland gubernatorial candidate, speaks to supporters during a campaign rally in Riva, Md. (Steve Ruark/AP)

REPUBLICANS ARE seeking to make Maryland’s gubernatorial race a referendum on the policies of outgoing Gov. Martin O’Malley (D), particularly his extensive record of tax increases. Fair enough: Although Mr. O’Malley isn’t on the ballot in November, his hand-picked protege — Lt. Gov. Anthony Brown — is, and there’s not much daylight between the two men on the issues or on the dozens of taxes and fees raised during the nearly eight years they’ve held office.

In their zeal to remind Marylanders of their tax burden, and blame it all on the O’Malley-Brown administration, the Republicans have fixed on a handy whipping boy that they delight in referring to as the “rain tax.” The Democrats are so tax-happy, they’ve even imposed a levy on the rain — or so goes the sound-bite-ready argument from the GOP gubernatorial candidate, businessman Larry Hogan.

It’s a good applause line, tailor-made for elements of a tax-weary electorate struggling with stagnant wages, stubborn unemployment and a sluggish post-recession economy. Harping on the supposed absurdities of the so-called rain tax may constitute the only promising angle of attack for Republicans in a state where they are outnumbered 2 to 1 among registered voters.

It’s a tougher sell on the merits. The “rain tax” is, in fact, a federally mandated levy on pollution caused by storm water run-off, one of the main culprits in the tragic, decades-long environmental degradation of the Chesapeake Bay. Established by state legislation passed in 2012, the tax applies to the state’s 10 most heavily populated urban and suburban jurisdictions, places with an abundance of hard surfaces — parking lots, roads, driveways. In those built-up places, storm water carrying sediment, nutrients, trash and a variety of other pollutants washes into nearby streams and rivers, which drain into the bay. Revenues from the tax are meant to help localities adopt programs and build infrastructure to limit the damage from that runoff in order to protect the body of water.

Among the Republicans who have mocked the rain tax is Texas Gov. Rick Perry, who suggested it should drive Maryland businesses to relocate to the Lone Star State. It may come as news to Mr. Perry and his fellow Republicans in Maryland, but states and localities across the country have used similar tools for years to respond to the problem of storm water-borne pollution. And it’s not as if Maryland rushed into imposing the tax; in fact, the legislation passed in 2012 was designed to comply with rules adopted by the federal Environmental Protection Agency.

If lawmakers in Annapolis had not devised a source of revenue dedicated to developing tools to contain and filter pollutants caused by run-off and floods, the money to deal with a growing problem would have had to come from general funds, meaning schools, parks, public safety and social services. Which of those budgets, exactly, would Republicans like to slash to provide the cash to manage the pollution and threat to the bay caused by storm water run-off? Mr. Hogan isn’t saying.