With Congress closing in on a final tax cut bill, 21 Republican governors from across the country sent a letter Thursday to House Speaker Paul D. Ryan (R-Wis.) and Senate Majority Leader Mitch McConnell (R-Ky.) asking them to finish the job.

“We urge the two chambers to pass meaningful tax reform legislation and send it to the President’s desk,” the letter said. “We’ve proven in our states that you can cut taxes, create jobs and generate budget surpluses all at the same time. If it can work in our states, it can work in America.”

There are 34 states with Republican governors, which raises the question: Why didn’t the other 13 sign the letter?

At least a handful object on policy grounds to parts of the legislation, while others appear to be sensitive to home-state politics — or some combination of both.

Several, for instance, preside over traditionally Democratic states, where the Republican tax legislation is particularly unpopular and where many residents could take a hit from provisions that curb the current practice of allowing state and local taxes to be deducted on federal filings.