The United States was once and is again becoming "a nation of immigrants." With Hispanics comprising both the largest minority and the fastest growing group in the United States, midterm election campaigns that focus on immigration reform could hold the key to winning over voters in many states.

After bottoming out in the 1970s, the immigrant influx has reached levels not seen since the 1930s, according to U.S. Census data.

Much of the focus is on the estimated 11.5 million to 12 million "unauthorized" immigrants -- comprising 3.8 percent of the U.S. population -- who have either overstayed temporary work visas or entered the United States illegally. The Pew Hispanic Center estimates the unauthorized immigrant population now grows by 500,000 people every year.

The availability of jobs is seen as the main force behind their arrival, but immigrants -- mostly from Mexico and Latin America -- also enter the country to reunite with family or escape persecution.

The Republican-controlled Congress and White House have been at odds over how to deal with the stresses immigrants create on America's education, health care and criminal justice systems.

In an address to the nation in May, President Bush outlined a five-point plan to reform the immigration process, including creating a temporary worker program, providing a path to citizenship for some illegal immigrants and increasing enforcement at the U.S.-Mexican border. President Bush was himself a border state governor -- from Texas -- from 1995 to 2000.

The Senate's version of immigration reform is a bipartisan plan close to President Bush's vision, but House Republicans have called these plans "amnesty" and have opted for a hard-line approach. Their legislation emphasizes enforcement and anti-terrorism, including the criminalization of illegal immigrants and the construction of a 700-mile-long border fence.

With the federal government unable to reconcile a comprehensive immigration policy, state legislatures have taken their own actions, now considering whether they should provide certain social benefits for illegal immigrants. Federal laws already ban illegal immigrants from receiving many benefits.

Many non-border states, such as those in the Southeast, have been forced to address illegal immigration now that their Hispanic populations are rising and significant numbers of unauthorized immigrants are working in farming and manufacturing jobs.

At the start of July, about 500 pieces of legislation had been introduced in state legislatures in 2006. Nine states advocating on illegal immigrants' behalf have passed legislation granting them in-state tuition rates. Eleven states allow illegal immigrants to receive driver's licenses or certificates.

Other states have cracked down on illegal immigration. The Colorado Legislature passed a bill in July that would deny illegal immigrants over the age of 18 most non-emergency state benefits. The governors of New Mexico, the state with the highest Latino population, and Arizona, the state with the toughest illegal immigrant laws, have declared a "state of emergency" to help them cope with crime caused by unauthorized migrants. Both governors, Bill Richardson, D-N.M., and Janet Napolitano, D-Ariz., are up for re-election.

"Immigration is the issue in Arizona," said Sen. Jon Kyl, R-Ariz., July 10 on the NewsHour with Jim Lehrer. "More than half of the illegal immigrants coming into the United States pass through Arizona. Some stay. And everyone here is aware of it. It is the big issue."

Democrats are hoping that protests in May over the House bill that brought out hundreds of thousands of immigrants and their supporters across the country could be an indication of a Hispanic backlash against Republicans that could help them win back House seats.

Latino Coalition surveys reveal that Democrats have been gaining ground with swing Hispanic voters: In December 2005, about 60 percent of Hispanic voters would have voted for a Democrat for Congress, and 21 percent for a Republican. The most recent survey found that immigration reform was the hottest issue with Hispanic voters.

Term length may play a role in the immigration reform strategies put forward by Republican members of the two chambers: Republican senators not up for re-election can appeal to the Hispanic community whose power continues to increase, while representatives, almost all of whom must defend their seats in the midterm election, may opt for an appeal to emotion in hopes of pushing disaffected Republican voters to the polls.

In California, the state with the most unauthorized immigrants, 2.5 million to 2.75 million, a special election held in June to replace imprisoned former Rep. Randy "Duke" Cunningham may signal how the immigration issue will play out in the midterm elections. Republican Brian Bilbray, who made immigration the focus of his campaign, won narrowly with results similar to those of the party divisions in the 2004 presidential election.

Meanwhile, the two major comprehensive reform bills considered in the House and Senate still need to be reconciled. This may not occur until after the Nov. 7 elections.

-- By Adnaan Wasey, Online NewsHour