Her company, All Seasons Landscaping, is in Colorado’s Sixth Congressional District, which wraps around Denver like a question mark. It has an unusually high proportion of college graduates and an unusually low unemployment rate; the average annual salary for workers who are not self-employed is nearly $63,000.

Landscape work is harsh. Digging in the dirt and heaving equipment in blistering heat produces aching backs and raw hands. Low-skilled workers can earn a similar wage making a sandwich or working in an air-conditioned warehouse.

“We put a $5,000 ad in The Denver Post, and we didn’t have one applicant,” Ms. Fox said. Paying a wage high enough to attract local workers would put her out of business, she said, because her customers would balk at the resulting price increases.

Like Ms. Fox and other landscapers, Mr. Steinhauer signed planting contracts with customers last year based on the assumption that his crews would earn roughly $15 an hour. “These are unskilled positions,” he said. “Would you pay $50 to plant a bush in your garden?”

“With the economy as good as it is, I don’t know many families who are telling their kids to become landscape laborers,” Mr. Steinhauer said. “And who wants to work a job where you get laid off in November and then come back?”

Creating jobs, particularly for neglected blue-collar working men, and reducing immigration have been at the center of President Trump’s agenda and a lodestar for his supporters. Lower jobless rates support the Republicans’ case that the economy is improving. And the increasingly hard line on immigration provides a framework for the administration’s policies on legal as well as illegal migrants.