As Mick Mulvaney prepared to step into the role as Donald Trump's Chief of Staff, reporters found audio of Mulvaney telling a radio host in 2015 that a wall wouldn't solve any of the country's immigration problems. Per the New York Post:
“The fence doesn’t solve the problem. Is it necessary to have one, sure? Would it help? Sure. But to just say build the darn fence and have that be the end of an immigration discussion is absurd and almost childish for someone running for president to take that simplistic of [a] view,” Mulvaney added, without making a distinction between a fence or wall.
...“And by the way, the bottom line is the fence doesn’t stop anybody who really wants to get across,” Mulvaney said in the interview.
Trump supporters say the president isn't wrong in his assessment—Democrat's unwillingness to come to the table to negotiate is prolonging the shut down. Per the LA Times:
Rep. Kevin McCarthy (R-Bakersfield), the House minority leader, said talks had been complicated because of Democrats’ hard-line stance, refusing to budge on the wall or even enter serious negotiations until the government is reopened.
“It is a real challenge when the Democrats won’t even give an offer back,” he said.
The Washington Post reports many of the same senators who are refusing to vote for Trump's wall voted to fund a barrier at the border back in 2013.
Every Senate Democrat voted for the Gang of Eight bill — including 36 Democratic senators still serving today. President Barack Obama agreed to sign it. Indeed, he praised the bill for including what he called “the most aggressive border security plan in our history” and said that “the Senate bill is consistent with the key principles for commonsense reform that I — and many others — have repeatedly laid out.” That bears repeating: Obama said building a 700-mile fence on the southern border was consistent with the principles of the Democratic Party.
Yet as Politifact points out, the barrier proposed in the 2013 legislation was fencing, much different than the concrete wall currently being proposed.
"It didn’t have anywhere near the gravity of harm," Angela Kelley, who in 2006 was the legislative director for the National Immigration Forum, told the Boston Globe. "It was hard to vote against it because who is going to vote against a secure fence? And it was benign compared with what was out there."