He consistently ranks as one of the most conservative members of the Senate, supporting tax cuts, smaller budgets and a federal ban on same-sex marriage. He opposed President George W. Bush’s bank bailout and the immigration overhaul Bush sought in 2007.

Then as now, Sessions said it would reward law breakers while providing few safeguards against future illegal immigration and hurting wages of lower skilled American workers. He got a boost last week from the Congressional Budget Office, which said the changes would slightly push down average wages for the first decade. “I am defending poor African-Americans, Hispanics and people of all kinds,” Sessions said.

Other detractors focus exclusively on the low hanging fruit of border security. Sessions hits the economic angle, too. “Nobody wants to talk about jobs, wages, unemployment,” he said. “You have to criticize Republican business interests.”

He sounds alarms not just about the 11 million people already living in the United States illegally but the many millions more that would enter legally if the bill is passed. He raises questions about a provision requiring illegal immigrants to pay back taxes, saying it would be next to impossible to determine who owes what, and scoffs at the $2,000 in fines people would have to pay over a number of years.

Sessions made those points repeatedly when the Judiciary Committee took up the bill last month, but his amendments were shot down. In one stinging rebuke, Sessions was on the losing end of a 17-1 vote to limit the number of immigrants who could legally enter the United States.

Undaunted, he has shown up on the Senate floor every day since the bill came up for general debate, carrying detailed notes. (On his office desk he keeps the durable Boy Scout motto: Be Prepared.) Glasses perched on the tip of his nose, he assails the bill as being worse than the 2007 version, often reaching for and waving around the thick printed copy that sits on his desk. Even so, he does it without the flamboyant outrage that draws the TV cameras. Reporters have increasingly ignored him as the debate wears on, seeing him as an outlier.

That seemed even more evident Thursday as Rubio and other lawmakers announced a major amendment that adds tens of billions in new border security spending, attracting more Republican support. To Sessions, it validated his work.

“Here in the middle of the debate, after the bill has been exposed, after it’s been hammered for failure after failure after failure after failure, they come up with a bill that says, ‘Don’t worry, now we’re going to throw 20,000 agents at the border and now you all have to vote for it,’ ” Sessions said.

“He knows his stuff and he’s a good lawyer,” said Sen. Orrin Hatch, R-Utah, who secured changes to the bill to favor the high-tech industry in his state, the kind of special interest handout Sessions says are laced throughout the bill. “I differ with him on a number of things but he’s totally honest,” Hatch continued. “He’s what a senator should be. Even if you disagree with him, he’s courteous and decent.”

Critics say Sessions is hiding his contempt for any kind of immigration reform behind the stated concern for poor workers. “His record shows no consistent opposition to bills that primarily benefit business owners or the wealthy, or that empower labor to wrest more economic gains from capital,” the liberal Washington Post writer Ezra Klein recently wrote, listing votes against the payroll tax cut and an extension of unemployment benefits.