Mike McGavick's latest radio ad is a politician's version of highway robbery.

The Republican candidate for U.S. Senate is appropriating the issue of sales tax deductibility as his own and using it to attack incumbent Maria Cantwell. Problem is, Democrat Cantwell might as well be known as Sen. Sales Tax Deductibility. Both she and Rep. Brian Baird, D-Vancouver, have been leaders, in their respective houses of Congress, on the issue of first restoring, then keeping, the right of residents in Washington and seven other states to deduct their state sales tax from the federal tax bill. For the 18 years previous, only state income tax could be deducted.

In 2004, Congress restored the sales tax deduction, which means an average of $550 for a family — but only for two years. Cantwell was co-author of the bill.

In February, Cantwell sponsored a bill to make the deduction permanent. It passed the Senate 75-25, and is before the House.

McGavick's radio ad attacks Cantwell for voting against the sales tax deduction — and Washington families. The deduction was embedded in a politically charged bill that also would have hurt some Washington families, especially tip workers.

It's an age-old political trick that reliably rears its head during election season: Roll popular elements into a controversial bill to get it passed. If it doesn't pass, challengers like McGavick can use it to attack the incumbent.

The sales tax deduction issue isn't over. When Congress reconvenes in September, the leadership should keep its previous promise to run this popular bill, unfettered by controversial issues.