Jeb Bush is mocking President Donald Trump as his plan to build a wall along the border between the U.S. and Mexico is running up against tough political and practical obstacles.

Bush, a former presidential candidate and Florida governor who took heat from Republicans for his relatively moderate stance on immigration, on Friday tweeted a link to a Wall Street Journal column that said Trump's plan is "easier said than done."

"Reality sets in," Bush wrote.

The piece, penned by GOP strategist Evan Siegfried, says the president has failed "to take into account the major hurdle the wall faces: eminent domain."

"To build the wall, the U.S. would need to own all 1,954 miles of the border," Siegfried writes.

He points out the U.S. government owns just 100 miles of the 1,254 miles of border land in Texas alone, and notes that the 2006 Secure Fence Act passed by Congress with bipartisan support to build a fence along 700 miles of the border still has significant gaps.

"The fence had to have holes to accommodate local ranchers whose cattle graze on the southern side, [and] due to property owners' fighting land seizures in court," Siegfried writes.

"How could Mr. Trump overcome this obstacle? He may be a great negotiator, but for logistical reasons alone it's unlikely he can personally make a deal with each of the hundreds, if not thousands, of landowners who'd turn to the courts to resist the seizure," he adds.

Also this week, conservatives in the House vowed to hold up additional funding to pay for the wall's construction unless the cost is balanced by cuts elsewhere.

"If all of a sudden we're not worried about pay-fors for our spending, then we have been hypocrites for six years," Rep. Raul Labrador, R-Idaho, said at a discussion on the wall with members of the House Freedom Caucus and the conservative Republican Study Committee on Tuesday. "So we need to make sure that whether it's a fence, whether it's the military, whether it's any other issue that we're discussing here in Congress… I'm not going to vote for anything that just increases spending without looking at a way to pay for that in the future."

CNN reported Thursday that a plan being prepared by U.S. Customs and Border Protection prefers a barrier more like the existing fence – that border agents can see through – rather than a solid wall.

And while the plan outlines a proposal to build a barrier covering the 1,080 miles of border where a fence is not already in place, they have given it a price tag of $21.6 billion.