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.

Among the blue state governors who took a pass on signing were Charlie Baker of Massachusetts, Larry Hogan of Maryland and Chris Christie of New Jersey.

Christie told reporters in his state Thursday that he’s still waiting to see a final version of the bill, while Hogan, who is up for reelection next year, didn’t respond to a request for comment. Democrats in his state have been trying to make political hay out of his near-silence on the GOP tax plans.

Baker has been critical of the bill in the past, zeroing in on a number of provisions he says would be detrimental to his state.

But some Republican governors kept their ink dry Thursday because of reasons unrelated to the legislation itself. Michigan Gov. Rick Snyder (R) said through a spokesman that he makes a practice of not signing on to multi-governor letters like this one.

And while Florida Gov. Rick Scott (R) didn’t sign the letter, aides were quick to point to a recent tweet in which the governor wrote: “Glad to see Washington is following Florida’s lead and finally taking action on cutting taxes.”

The letter was orchestrated by Wisconsin Gov. Scott Walker (R), whose staff was tasked with gathering the signatures of GOP colleagues.

An aide to Walker said the governor’s office was pleased with the effort, given the logistical challenges of collecting signatures from statehouses across the country. The Walker aide, who is not authorized to speak for the governor and requested anonymity, characterized 21 signatures as “a pretty strong number” for such an exercise.

As for those who didn’t sign, “a good chunk of them will only sign letters they send themselves.”

Jon Thompson, communications director for the Republican Governors Association, echoed those points, adding that his organization didn’t spearhead the collection of the signatures.

Jared Leopold, Thompson’s counterpart at the Democratic Governors' Association, said the 13 GOP governors who didn’t sign the bill was evidence of a “huge divide in the Republican Party over this legislation.”

“Republicans who are up for reelection are scared of their shadow and realize this bill is very unpopular,” Leopold said, adding that Democrats plan to press the issue in upcoming elections.

Several recent polls have shown strong majorities of Americans opposed to the legislation, with most saying it will disproportionately benefit corporations and the wealthy. President Trump has promised to sign a bill by Christmas after it emerges from a House-Senate conference committee.

Ohio Gov. John Kasich, among those who competed for the GOP nomination against Trump last year, has been among the more outspoken critics of the tax bill.

Kasich, who didn’t sign the letter sent Thursday, has said it would lead to “an explosion in the debt which would be counterproductive to economic growth.” He has also criticized the Senate for including a provision that would repeal the individual mandate in the Affordable Care Act.

Other Republican governors who didn’t sign the letter include Nathan Deal of Georgia, C.L. “Butch” Otter of Idaho, Bruce Rauner of Illinois, Sam Brownback of Kansas, Brian Sandoval of Nevada, Phil Scott of Vermont and Jim Justice of West Virginia.

An aide to Otter cautioned against reading too much into his absence.

“We are still doing our due diligence on the letter and vetting our response or support through our tax policy experts,” said spokesman Jon Hanian. “We need to do our own homework on it.”

An aide to Brownback attributed the absence of his signature to a focus on preparing for a Trump administration position.

“Governor Brownback is awaiting confirmation to be the U.S. Ambassador at Large for Religious Liberty and has not been engaging with official letters such as the one sent to Majority Leader McConnell and Speaker Ryan,” said Rachel Whitten, Brownback's spokeswoman.

Ovetta Wiggins contributed to this report.