By DAVE MONTGOMERY
McClatchy Newspapers

For all their differences, Barack Obama and John McCain have often shared common ground on one volatile issue: immigration.

McCain was the architect of legislation to legalize millions of undocumented immigrants, a concept Obama also has embraced.

However, the two presidential candidates have made immigration the centerpiece of hard-hitting Spanish-language radio and TV ads, with each candidate presenting himself to Hispanics as the true champion of comprehensive immigration reform.

The ad slugfest is under way in four battleground states - Florida, Colorado, New Mexico and Nevada - and underscores the importance of Hispanic voters as well as the political punch that immigration carries in the Latino community.

The uproar over illegal immigration has faded amid voter anxiety over the nation's economy. However, it remains an urgent concern to many and could return to the forefront when the next president is forced to confront an issue that's vexed the nation for at least two decades.

Frank Sharry, the executive director of America's Voice, which advocates comprehensive immigration legislation, said the ad blitz is targeted at hundreds of thousands of newly registered Hispanic voters "who are arguably are going to be the swing vote in the swing states that could decide who the next president is."

One McCain ad tells voters that the Arizona senator "stood up" for Hispanics with legislation to overhaul the nation's immigration laws. It also claims that Obama supported "poison-pill" amendments designed to kill the bill, even though the Illinois senator in fact supported the legislation pushed by McCain and Sen. Edward M. Kennedy, D-Mass.

In turn, Spanish-language ads from the Obama camp attack McCain for "distortions and lies" on immigration, charging that the Republican nominee caved in to anti-immigration forces within the Republican Party.

Like the rest of the electorate, Hispanics consider the nation's economic crisis to be their top domestic worry, but immigration still ranks relatively high among their issues of concern. While McCain traditionally has polled well among Hispanics, in part because of his stewardship of the 2005 McCain-Kennedy bill, a mid-summer poll by the Pew Hispanic Center showed that Hispanics favored Obama over McCain by 66 percent to 23 percent.

At one point, the number of undocumented immigrants was estimated at more than 12 million, but the illegal population has shrunk at least slightly, experts say, partially as a result of an economic downturn that's made the U.S. less attractive as a bountiful source of jobs.

In October, the Pew Center estimated the undocumented population at 11.9 million. From 2000 to 2004, approximately 800,000 illegal immigrants a year crossed the U.S.-Mexican border in search of work, but over the past three years, the average has fallen to 500,000 a year, Pew reported.

McCain and Obama appear to be in basic agreement on the most controversial aspect of immigration. Both advocate a path to citizenship for those in the country illegally if they meet certain conditions. Those who qualify, however, would be required to go to the "back of line" behind other applicants for legal residency.

Obama would require undocumented immigrants to pay a fine and learn English. McCain would impose those requirements too, and he thinks that illegal immigrants should pay back taxes and pass a citizenship course.

Until the onset of the 2008 presidential race, McCain won accolades among pro-immigrant groups for the McCain-Kennedy bill and his support of Bush's initiatives. But some pro-immigration leaders are taking a second look after McCain, as a candidate, retooled his immigration strategy to insist on securing the U.S. borders before moving forward with legalization and other aspects of immigration reform.

"It's not clear which John McCain is going to step forward on immigration if John McCain becomes president," said Angela Kelley, the director of the Immigration Policy Center.

On his campaign Web site, McCain outlines a two-step process to reforming the "broken" immigration system, beginning with the "top priority" of securing the borders through the use of physical and virtual barriers, unmanned aircraft, beefed-up enforcement and other measures. Border-state governors would be required to certify that their borders are secure, though McCain's plan doesn't specify how.

Obama says he, too, wants to "preserve the integrity of our borders" through additional personnel, infrastructure and technology on the border and at ports of entry, but he hasn't made border security a pre-condition for legalization and other elements of his immigration plan.

McCain and Obama both voted for a controversial 700-mile border fence that the Bush administration is racing to complete before the end of the year. Both also support an electronic verification system to ensure worker eligibility and vow to aggressively crack down on employers who hire illegal immigrants.

McCain has updated his three-year-old proposal for a guest worker program that would create a steady flow of unskilled workers to fill jobs generally bypassed by U.S. workers. The original McCain-Kennedy bill called for up to 400,000 workers a year, depending on need. McCain's latest proposal sets no numerical limits and would tie the yearly flow to the rise and fall of market demand.

Workers would be required to return home after their temporary visas expired, but a limited number of green cards - the first step toward citizenship - would be made available to workers who want to stay in the country permanently. McCain says the number would be small.

Obama has voted for a guest-worker program but doesn't offer details for a guest-worker program on the Obama-Biden Web site, other than recognizing the need "to increase the number of people we allow into the country legally to a level that keeps families together and meets the demand for jobs that employers cannot fill."

(Montgomery reports for the Fort Worth Star-Telegram.)