PHOENIX (AP) - The federal lawsuit against Arizona's tough new

immigration law focuses heavily on a question that has been in the

spotlight repeatedly the past decade and dates back to the Founding

Fathers: The right of the government to keep states from enacting

laws that usurp federal authority.

The lawsuit filed in Phoenix federal court on Tuesday

sidestepped concerns about the potential for racial profiling and

civil rights violations most often raised by immigration advocates.

Experts said those are weaker arguments that don't belong in a

legal challenge brought by the White House to get the measure

struck down.

Instead, the suit lays out why the government believes that

immigration laws passed by Congress and enforced by a range of

federal agencies must take precedence to any passed by a state

Legislature.

The Arizona law requires officers, while enforcing other laws,

to question a person's immigration status if there's a reasonable

suspicion that they are here illegally, such as speaking poor

English, traveling in an overcrowded vehicle or hanging out in an

area where immigrants typically congregate.

The law also makes it a state crime for legal immigrants to not

carry their immigration documents.

Backers of the law say the crackdown is a necessary tool to keep

illegal immigrants out of Arizona and combat problems such as drug

trafficking, murders and violent kidnappings that have become so

common in a state that is home to an estimated 460,000 undocumented

residents.

The federal government will ask a judge to grant an injunction

to block the law from taking effect on July 29.

The arguments will focus on a core constitutional concern -

balancing power between the states and the federal government. More

specifically, the issue centers on the long-running "pre-emption"

legal argument that says federal law trumps state law.

"The nation's immigration laws reflect a careful and considered

balance of national law enforcement, foreign relations, and

humanitarian interests," the suit says.

The lawsuit goes on to say that a "state may not establish its

own immigration policy or enforce state laws in a manner that

interferes with the federal immigration laws. The Constitution and

the federal immigration laws do not permit the development of a

patchwork of state and local immigration policies throughout the

country."

Backers of the law say that Arizona will have some strong

arguments in its favor in fighting the lawsuit.

Kris Kobach, the University of Missouri-Kansas City law

professor who helped draft the Arizona law, has said the state law

is only prohibiting conduct already illegal under federal law. And

Harvard Law School professor Gerald Neuman believes Arizona could

make a compelling legal argument that it has overlapping authority

to protect its residents.

But courts have ruled that under the Supremacy Clause of the

Constitution, any state law that conflicts with a federal law is

pre-empted. Federal law, the framers said, "shall be the supreme

law of the land."

The pre-emption tactic has been successfully used by the federal

government on several occasions over the years, including by the

Bush administration to limit product liability lawsuits. The

government also used it to overturn bans on military recruiters

passed by liberal California towns.

Federal courts have invoked the Supremacy Clause on immigration

issues as well. For example, a federal judge in 2008 struck down a

Dallas suburb's ordinance that banned apartment rentals to illegal

immigrants, saying the U.S. government has the ultimate authority

to enforce immigration laws.

Within months of taking office, the Obama White House directed

department heads to undertake pre-emption of state law only with

full consideration of the legitimate prerogatives of the states.

The 2009 directive was aimed at reversing Bush administration

policy which had aggressively employed pre-emption in an effort to

undermine a wide range of state health, safety and environmental

laws.

Despite the precedent, that doesn't mean the lawsuit is a sure

winner, or that state officials don't believe they can pass laws

that head into federal turf.

In fact, efforts by many states trying to block the nation's new

health care law run headlong into the Constitution's Supremacy

Clause. But immigration is one area where federal authority has

generally been upheld.

"Immigration has traditionally and constitutionally been the

historic preserve of the federal government, and there are cases

going back to the late 19th century that say as much," said Peter

Spiro, a constitutional law professor at Temple University who has

studied immigration law extensively. "So the Obama Administration

has a lot to work with in filing this claim, and the fact that the

claim is filed by the administration adds credibility ... and

increases the chances that law will be struck down on pre-emption

grounds.

"That said, it not by any means a slam dunk," Spiro said.

Regardless of how the case is determined at the district court

level, it will likely be appealed. The U.S. Supreme Court is

already set to hear an Arizona immigration case in the fall when it

takes up a challenge to a 2007 state law punishing employers who

knowingly hire illegal immigrants.

If the high court doesn't issue a broad ruling on states' rights

to implement laws on immigration in that case, prepare to see the

case filed Tuesday make to the justices, Spiro said.

"It's clearly an important case. The Arizona law is

unprecedented in its aggressive posture towards illegal immigrants.

It's an important issue federally, really, that's the way the

administration is framing it. They say the states do not have this

kind of role as far an immigration legislation."

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