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I don't want arguments for or against the wall/barrier on the United States' southern border. I would like to understand what Nancy Pelosi means when she or any other member of the Democratic party claim that a wall is immoral.

Here are some examples of statements by Democratic politicians and my best guess at interpretation:

Speaker of the House called President Trump's planned border wall 'immoral'

“A wall, in my view, is an immorality. It’s the least effective way to protect the border and the most costly. I can’t think of any reason why anyone would think it’s a good idea — unless this has something to do with something else.”
Source

What Speaker Pelosi seems to be saying here is that the border wall is immoral because it's costly but it's not even effective, and would therefore be immoral waste of taxpayer money.

Congresswoman Andrea Ocasio-Cortez stated that all Latinos wherever they're from have the right to enter the United States, so in her view, not letting them enter could be perceived as a violation of their rights, which could be considered immoral.

Rep. Adam Smith, the new chair of the House Armed Services Committee, who, echoing many other Democrats, claims that Trump’s campaign for a border wall is rooted in “xenophobia and racism.”
Source

So in Rep. Adam Smith's view, if the border wall is rooted in 'xenophobia and racism', then the border wall is immoral because xenophobia and racism are immoral.

Rep. Veronica Escobar, D-Texas, stated

“I hope folks have realized walls are deadly,” she said. “When people are so desperate they will flee their country to go to another, a wall doesn’t stop them. It just pushes them out to more treacherous, dangerous, deadly crossings where migrants die in the desert or drown in rivers.”
Source

So, in Rep. Escobar's view, the wall would be immoral because it would make migrants' journeys more dangerous and expose them to potentially greater harm that if there weren't a wall.

People say it's immoral in its imagery, in that a wall is an "insult" to those living outside the U.S.

People also argue that not building the wall could be immoral, because not having one makes it more tempting to migrants willing to take the dangerous path across the wilderness/desert.