President Trump's plan to build a wall on the southern border and force Mexico to pay for it isn't viable, Sen. John McCainJohn Sidney McCainThe Hill's 12:30 Report Senate gets to work in August — but many don’t show up Rand Paul’s Russia visit displays advancement of peace through diplomacy MORE (R-Ariz.) said in a new interview.

McCain — the chairman of the Armed Services Committee — told CNN that Trump's plan is "not a viable option." 

"If you only build a wall, only a 'wall,' without using technology, individuals, drones, observations, etc., you're not going to secure the border," he said.
 
McCain isn't the only Republican speaking out. 
 
Sen. John CornynJohn CornynSen. Warner to introduce amendment limiting Trump’s ability to revoke security clearances Sentencing reform deal heats up, pitting Trump against reliable allies Rand Paul to ask Trump to lift sanctions on Russian leaders MORE (R-Texas) told The Hill last month that he agrees with Trump that "we need to secure the border" and that a physical barrier would be an "important part of it." 
 
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"But it's also going to take technology, ground sensors, radar ... and then it's going to take trained border patrol," he said. 
 
He added to CNN that he also has concerns about "spending un-offset money, which adds to the debt, period."
 
Estimates on the cost of building the wall have run from $10 billion to more than twice that amount.
 
"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," Cornyn said.
 
Sen. Orrin HatchOrrin Grant HatchSentencing reform deal heats up, pitting Trump against reliable allies Dem lawmaker calls Trump racist in response to 'dog' comment PETA calls out Trump for attacking Omarosa as a 'dog' MORE (R-Utah) also said earlier this week that he has questions about a floated plan to include a tariff on imports from Mexico to pay for the wall.
 
“We don’t have definitive answers to any of those questions at this particular point, and without them, I don’t think I can give definitive positions on the proposal,” Hatch said during a Chamber of Commerce speech.
 
Sen. James Lankford (R-Okla.) told CNN that he wouldn't "count on Mexico to pay for our national security."
 
House Republicans in border districts are signaling that they are leery of the plan to build a wall through their districts. Three GOP lawmakers say undertaking such a massive project will fall short of alleviating the issues surrounding border security.
 
 
He noted that the Senate had previously passed legislation that would include a number of options, acknowledging that "it's a long border and varied border and what makes sense in one area doesn't make sense in another." 
 
Trump hasn't shown signs of walking away from the wall. 
 
He has already signed an executive order calling for its construction, and Homeland Security Secretary John Kelly told Fox News that he hoped to have the wall completed within two years.

White House press secretary Sean Spicer floated the idea of a 20 percent tax on imports from Mexico last week, but later clarified that such a move is just one option the White House is considering.