WASHINGTON – In the Senate, they're being called “triggers,” steps designed to seal the border and end illegal immigration. Whether they can is an open question, but their role in the debate unfolding on Capitol Hill is vital. Both sides view them as crucial to the outcome.

Backers of a compromise bill hope they can dispel public concern about future illegal immigration as the Senate considers green cards for millions of illegal immigrants.

They point out that the triggers include several steps long sought by the “get tough” crowd – the hiring of more Border Patrol agents, more border fencing and ways to help employers know for sure whether prospective employees are eligible to work in the United States.

Skeptics say triggers have been deliberately miscast to the public as silver bullets. The result, they argue, could be another overhaul that fails to end illegal immigration while ratcheting up legal immigration.

“These triggers have no teeth,” said Sen. Jeff Sessions, R-Ala. “I don't think they'll do much at all.”

Under the compromise, as amended last week by the Senate, illegal immigrants would get a probationary card that would allow them to remain and work in the country indefinitely. But certain milestones would have to be met before any green cards – permanent visas – could be issued.

The Border Patrol would have to grow to 20,000 from its current level of 14,000. The fence along parts of the U.S.-Mexico border would have to be extended to 370 total miles. More detention space, ground sensors and vehicle barriers would be required. And an employment verification system would need to be established.

The federal government has a long history of throwing money at border enforcement with little effect, spending tens of billions of dollars over the past three decades.

In the 1990s, the Clinton administration sharply ratcheted up resources for the border, spending at a record level to add agents, detention beds, prosecutorial staff and investigators. With the support of the National Guard and a push by Republicans in Congress, fences were built along the border, including in San Diego County. Stadium-style lights, ground sensors and remote-control video cameras were installed. Helicopters, balloon radars, night-vision equipment and more detention space were provided.

Perhaps the most dramatic change was the way Border Patrol agents were arrayed. Traditionally, agents waited back from the border for the immigrants to cross en masse. The agents would then chase swarms of people in a game that proved counter-productive. For every illegal crosser caught, two got away, experts estimated.

In September 2003, the Border Patrol began experimenting with a new strategy. It placed agents in fixed positions at heavy crossing areas. The around-the-clock presence was a powerful deterrent, but it shifted the flow to other parts of the border where monitoring was less intense.

Despite those aggressive efforts, the nation's illegal-immigrant population has grown each year. That growth accelerated from 400,000 a year in the early 1990s to the current estimated net growth of 700,000 illegal immigrants a year. The illegal population in the United States is estimated at 12 million and increasing.

Proponents of the Senate compromise bill and its triggers say the goal is to stop illegal immigration and make future legalization programs like the one being considered unnecessary.

For example, Sen. John McCain, R-Ariz., who helped craft the legislation, said its trigger requirements would “finally accomplish the extraordinary goal of securing our borders.”

But what if illegal immigration continues as it has for three decades even if all the steps are taken and all the triggers are met?

“Then we'll know we haven't done enough,” said Sen. John Kyl, R-Ariz., a supporter of the compromise.

Meanwhile, critics argue, millions of probationary and green cards will have been given out.

It reminds critics of what happened 21 years ago when Congress passed the landmark Immigration Reform and Control Act in 1986. It gave amnesty to 2.7 million illegal immigrants in exchange for criminal sanctions against employers who hire illegal immigrants. The employer sanctions were supposed to stop illegal immigration.

But illegal immigration accelerated. Employers and workers got around the sanctions with widely available phony documents, including driver's licenses and birth certificates.

Hence the importance of the final trigger: a fraud-resistant system to verify that noncitizens are eligible to work in the United States. But the Senate bill says only that a system has to be established, not that it has to be successful or even used.

Any meaningful “employment verification system,” critics fear, will be resisted by business groups and other immigrant advocates. A decade ago, when a bipartisan congressional commission recommended a national worker registry as the most feasible fix for employer sanctions, the resistance was fierce.

Even steps to combat document fraud by setting basic national standards for the appearance of driver's licenses and birth certificates were vigorously opposed.