Importantly, it is not solely empirical evidence on the effects of Walmart that informs conservatives here, but a general optimism about the benefits of competition. This optimism flows from a belief that while competition leads to jobs and businesses being destroyed, markets will reallocate the workers and capital, and in the long-run we will all be richer for embracing a dynamic and competitive economy.
However, too many Republicans abandon this optimism about the benefits of competition when it comes to immigration. Suddenly, economic pessimism replaces economic optimism. They focus on those who compete directly with immigrants and as a result lose their jobs and have lower wages. The most important issues quickly become concerns about how the economy will adjust to congestion, overcrowding, and pollution.
Lost in such pessimism is the usual conservative optimism about the benefits of when the government gets out of the way and lets people work and trade together in free markets. Gone is the optimism about how competition helps create a dynamic and growing economy, benefits consumers, and makes all of us, even the employees of Mom & Pop groceries, better off in the long-run. Many Republicans forget that trade and free markets are not a zero sum game; that immigrants pay taxes, buy goods, innovate, start companies, and grow the size of our economic pie. There's empirical evidence backing all this up, but Republicans are often drawn to the most pessimistic studies and interpret all of the evidence in light of strong prior beliefs about a zero sum economy where competition is a bad thing.
On top of their unconservative pessimistic assessment of the effects of immigration, Republicans also toss aside the conservative principle of limited government when it comes to what to do about these effects. Normally, conservatives realize it is difficult to claim the mantle of limited government while simultaneously supporting the government goal of restricting competition. You can see this principle reflected clearly inconservatives' support for free trade, opposition to public sector monopolies, and suspicion of regulations. But such principles are quickly abandoned when it comes to immigration. Take for example the recent Senate testimony of Jan Ting, a fellow at the Center for Immigration, who worried that amnesty for illegal immigrants "would make these immigrant workers better able to compete with American workers for jobs, including at least 12 million American workers still trying to find work". Why should Mom & Pop grocery stores, public sector workers, and manufacturers losing market share to China not be shielded from competition, but those competing with immigrants should? It is unconservative and inconsistent for Republicans to embrace this competition stifling goal for the government.