Last weekend, 2012 Republican presidential nominee Mitt Romney and his wife, Ann, were in Washington, where they met with friends and supporters and attended the annual Alfalfa Club dinner, an exclusive black-tie event.

One of the more memorable lines from last year’s closed-to-the-media dinner was former Florida Gov. Jeb Bush’s needling of his most famous sibling, when — while addressing the then-unsettled GOP nomination contest — he reported that at one point his brother even thought about running again: “‘George,’ I said, ‘the Constitution prohibits you from running again.’ He said, ‘Wow, they put my name in the Constitution?’”

The weekend’s real political news, however, was being made by the GOP’s 2008 standard-bearer, Sen. John McCain of Arizona, along with Republican Senate colleagues Jeff Flake of Arizona, Lindsey Graham of South Carolina and Marco Rubio of Florida, all of whom had been working with Democratic Sens. Michael Bennet of Colorado, Dick Durbin of Illinois, Bob Menendez of New Jersey and Chuck Schumer of New York to produce a comprehensive compromise immigration plan.

That plan — which said that after measures were taken to secure the border, undocumented immigrants could obtain legal status on a probationary basis once they had undergone a background check, and paid a fine and any back taxes owed — was proudly unveiled by the eight senators.

By losing the growing Hispanic vote nationally 71 percent to 27 percent to President Barack Obama after running a fierce get-tough-on-illegal-immigrants campaign, Romney not only made the senators’ bipartisan immigration proposal possible. For his fellow Republicans, Romney made the switch on immigration imperative.

As McCain put it Wednesday at a breakfast hosted by Politico, “If you have a large bloc of Americans who believe you’re trying to keep their … fellow Hispanics down and deprive them of an opportunity, obviously that’s going to have an effect.”

What sort of an effect? McCain: “The trend will continue of lack of support from Hispanic voters, and also, as you look at the demographics of states like mine, that means we will go from Republican to Democratic over time.”

One irony is that McCain was one of Romney’s most important 2012 endorsers for the nomination. This surprised some of us who remembered the Romney TV spot four years earlier that charged that McCain had “pushed to let every illegal immigrant stay here permanently — even voted to allow illegals to collect Social Security. And Mitt Romney? He opposes amnesty for illegals.”

Earlier in 2007, Romney test-drove his 2012 campaign when he attacked an Arkansas plan that enabled the children of undocumented immigrants to apply for college scholarships. Former Arkansas Gov. Mike Huckabee counter-punched: “I’m standing here tonight on this stage because I got an education. If I hadn’t had the education, I wouldn’t be standing on this stage, I might be picking lettuce. In all due respect, we’re a better country than to punish children for what their parents did …”

In 2008, Romney got even tougher on Texas Gov. Rick Perry and the Lone Star State’s Dream Act: “One of the things I still can’t get over is the idea that a state would decide to give $100,000 discounts to the children of illegals to go to school in their state.”

According to projections from the respected Pew Research’s Social & Demographic Trends, Hispanics, who constituted just 3.5 percent of the U.S. population in 1968 and 17 percent in 2011, will jump to 29 percent of the U.S. total in 2050. Asian-Americans, who were a tiny 0.6 percent in 1960, will increase to 9 percent. With African-Americans growing to 13 percent of the nation, this would mean that whites — who had been 85 percent of all Americans in 1960 — would shrink by the middle of the century to approximately 47 percent.

A bipartisan immigration bill staking out the path to citizenship will not, by itself, be a political silver bullet for a GOP still trying to shake off the anti-immigrant baggage the Romney candidacy saddled it with. But it could mean that Republican candidates might eventually, after hard work, hope to reach the 40 percent of the Hispanic vote George W. Bush won in 2004.

Mark Shields is one of the most widely recognized political commentators in the United States. The former Washington Post editorial columnist appears regularly on CNN, on public television and on radio. Click here to contact him, or click here to read previous columns. The opinions expressed are his own.