As another political firefight erupts over illegal immigration, it's easy to forget how recently a bipartisan solution to the incendiary dilemma appeared within reach.

Just four years ago, 62 U.S. senators, including 23 Republicans, voted for a comprehensive immigration reform bill that included a pathway to citizenship for illegal aliens. That bill was co-authored by Arizona Republican John McCain and Massachusetts Democrat Edward Kennedy. President Bush strongly supported it. The Republican supporters also included such conservative senators as Sam Brownback of Kansas and Mitch McConnell of Kentucky. The 39 Democratic supporters included a freshman senator from Illinois named Barack Obama.

Reform advocates suspect that Graham is withdrawing from the immigration effort partly to avoid embarrassing his close ally McCain.

That bill offered a three-step approach to reform that remains the most plausible template for consensus. It would have toughened enforcement of immigration laws, devoting additional resources to guarding the border and policing employers who hire undocumented workers. It established a guest-worker program to regulate the flow of immigrant labor. (Under an Obama amendment, that guest-worker program would be suspended whenever unemployment reached 9 percent.) And it provided a pathway to citizenship for illegal immigrants who pass a background check, pay a fine, and learn English.

The bill attracted substantial support from business, religious, and civil-rights groups. The measure almost certainly could have attracted the necessary 218 votes to pass the House. But it died when House GOP leaders refused to bring it to a vote because they concluded that it lacked majority support among House Republicans.

Since 2006, Republican support for comprehensive action has unraveled. In 2007, Senate negotiators tilted the bill further to the right on issues such as border enforcement and guest workers. And yet, amid a rebellion from grassroots conservatives against anything approaching "amnesty," just 12 Senate Republicans supported the measure as it fell victim to a filibuster. By 2008, McCain declared in a GOP presidential debate that he would no longer support his own bill: Tougher border enforcement, he insisted, should precede discussion of any new pathway to citizenship.

That view has since solidified among Republicans. For months, Sens. Lindsey Graham, R-S.C., and Charles Schumer, D-N.Y., have been negotiating an enforcement-legalization plan that largely tracks the 2006 model with some innovative updates, including a "biometric" Social Security card to certify legal status for employment. On balance, their proposal appears more conservative than the 2006 bill.

Yet it has been stalled for weeks because Graham had demanded that a second Republican sign on as a co-sponsor before the legislation is released, and none stepped forward. Even Graham angrily backed away this week, after Senate Democratic leaders briefly suggested they would move immigration reform ahead of climate-change legislation he is also negotiating. Reform advocates suspect that Graham is withdrawing from the immigration effort partly to avoid embarrassing his close ally McCain, who faces a stiff primary challenge from conservative former Rep. J.D. Hayworth. Meanwhile, McCain, continuing his headlong retreat, has praised the draconian law that Arizona Gov. Jan Brewer signed last week to require police to verify the immigration status of anyone they suspect to be in the state illegally.

As Franklin D. Roosevelt moved to the left during his second term as president, one Southern Democrat declared, "The Roosevelt of 1938 does not appear to be related by blood or marriage to the Roosevelt of 1933." The same could be said about John McCain on immigration since 2006 -- and indeed about the GOP overall. Bush's vision of courting Hispanics with a balanced approach that acknowledges the impracticality of deporting millions of people has surrendered to an insular enforcement-first strategy. Although some allies in the Bush effort have criticized Arizona's harsh law, other Republicans, including two gubernatorial candidates in neighboring Nevada, have already pledged to replicate it. They probably won't be the last.

Amid drug violence in Mexico and high unemployment in the U.S., concern about controlling the borders is understandable. But the hardening GOP position also shows how the party is being tugged toward nativism as its coalition grows more monochromatic: In a nation that is more than one-third minority, nearly 90 percent of McCain's votes in the 2008 presidential election came from whites. That exclusionary posture could expose the GOP to long-term political danger. Although Hispanics are now one-sixth of the U.S. population, they constitute one-fifth of all 10-year-olds and one-fourth of 1-year-olds. The larger threat is to America's social cohesion. Democrats, with their own divisions, can't reform the immigration system alone. Either both parties will accept that responsibility or the nation will likely suffer through years of sharpening social division symbolized by the escalating battle over Arizona.

This article appeared in the Saturday, May 1, 2010 edition of National Journal.

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