Spending offsets and questions about how effective a barrier a wall might be are causing problems for Donald Trump's proposal to build a wall along our southern border.

What's most worrisome to Trump supporters is that there appears to be a growing number of Republicans who might support the concept of a wall but are looking to offset its $12-billion cost with spending cuts elsewhere in the budget.

CNN:

Sen. John Cornyn, the No. 2 Senate Republican who represents the border state of Texas, was deeply skeptical about whether a wall alone would be enough to deter immigrants from entering the country illegally. And he issued a stark warning to Trump.

"I have concerns about spending un-offset money, which adds to the debt, period," Cornyn said bluntly when asked about the wall. "I don't think we're just going to be able to solve border security with a physical barrier because people can come under, around it and through it."

Trump has increasingly said that Congress would fund the wall initially but would later be reimbursed completely by Mexico. But prominent Republicans say flatly that they don't think Mexico will pay the United States back -- and Mexican leadership has repeatedly said the same -- meaning that taxpayers would be left holding the bag.

"No," Arizona Republican Sen. John McCain said when asked if he thought Mexico would reimburse the United States for the wall. "It's not a viable option."

McCain added: "If you only build a wall, only a 'wall,' without using technology, individuals, drones, observations, etc., you're not going to secure the border."

Trump still has time to convince his party to go along, especially if he provides more details on his plans, given Republicans are largely united behind calls for more robust border security. And some GOP senators, like Ron Johnson of Wisconsin, suggest they're open to paying for the wall even if it's not offset by spending cuts.

Behind the scenes, the Trump administration has tried to shore up support. John Kelly, the new secretary of Homeland Security, began to discuss with lawmakers this week his border security plans, sources said, but senators reported there were few details on the specifications of the wall.

The Trump administration is working on a supplemental funding package to pay for border and national security, but that work has stalled as the Senate moves slowly to confirm Trump's pick of Rep. Mick Mulvaney as his budget director.

I don't think the wall plan is in serious jeopardy.  The president could propose a package of budget cuts and revenue enhancements by taxing Mexican products at the border.  Or, as Speaker Ryan suggests, he can appeal to budget hawks by making the wall a national security priority.

The GOP has been talking border security for eight years.  To fail to pass the first concrete measure to reduce the flow of illegals would be a political mistake of the first magnitude.  The wall will get built – for better or worse – and Republicans will be virtually united behind funding it.