WASHINGTON — The candidates brandished a “Health Care Bill of Rights for Seniors” that promised to protect Medicare from their opponents’ “raid.” The party chairman wrote an op-ed warning that the other guys’ plan meant that “senior citizens will pay a steeper price and will have their treatment options reduced or rationed.” The ads were even blunter. “You cut our Medicare,” one accused. “And this November, you’re fired.”

Democrats in any election you care to name? Nope. It was Republicans running against the Affordable Care Act’s Medicare cuts in 2010. It worked, too. Seniors turned out in big numbers, and Republicans carried voters over 65 by an astounding 21 points — by far their biggest margin among any age group.

The GOP’s Medicare attack was a smart play that capitalized on an important demographic shift in U.S. politics. In recent years, the Grand Old Party has been getting even older. The AARP crowd was the only age group that supported John McCain over Barack Obama in the 2008 election. That was also the first election in decades in which the average Republican voter was older than the average Democrat.

At the same time, the GOP is losing support among the young. Voters between 18 and 29 were the only age group that didn’t break for Republicans in 2010. In 2008, Obama won them by 34 points. So Republicans aren’t courting seniors — who turn out at the highest rate of any age group — just because they want to. They’re doing it because they have to.

But in 2012, this alliance will be tested. In 2010, President Obama’s health-care law gave Republicans a way to unite their passionate opposition to the legislation with seniors’ passionate opposition to cutting Medicare. At the time, many people noted the peculiarity of the Republican Party remaking itself as the guardian of Medicare and wondered if it could last. Now we know it couldn’t.

The 2012 budget proposed by Rep. Paul Ryan, R-Wis., and passed by House Republicans on Friday would privatize Medicare and shift most of the entitlement’s future costs onto seniors. According to the Congressional Budget Office, come 2030, seniors would be paying for nearly 70 percent of their private Medicare-certified insurance out of pocket. If the program were left alone or reformed another way, they’d only be paying 25 to 30 percent.

Ryan has considered this problem and chosen to exempt everyone over age 55 from his plan and delay implementing it for a decade. But since he needed savings now, he adopted all of the Affordable Care Act’s Medicare cuts — the very cuts his party opposed in 2010. There’s courage in that move, as many of those savings are wise, but there’s also rank hypocrisy for, well, the same reason. Beyond that, there’s risk. Republicans are relying on the senior vote like never before. And they might have just put it into play.

A Gallup poll released Wednesday underscored the tension in the Republican coalition. It asked Americans whether they “think the government should completely overhaul Medicare to control the cost of the program, make major changes to Medicare but not completely overhaul it, make minor changes to Medicare, or should the government not try to control the costs of Medicare?” Among Democrats, “minor changes” was most popular, followed by “major changes.”

You might assume that Republicans would be a lot friendlier to remaking the program. Not so. Among Republicans, “not try to control costs” was the most popular position, followed by “minor changes.” And to call Ryan’s plan anything less than a complete overhaul would be to insult it.

“This is 1995 all over again,” Robert Blendon, a professor of health policy and political analysis at Harvard, said of the GOP. “They’re in a good position, they win, and then suddenly they’re going to try and completely change Medicare. But though the leadership of the party is very excited about that, there’s no support for it in the polls.”

That gets to the fundamental problem that the GOP faces. When seniors tilted toward the Democrats, their interests aligned: Seniors love Medicare and Social Security, and so do Democrats. Republicans don’t. They have an ideological objection to both programs, and would like to see them partially or fully privatized. And so, as we saw with President George W. Bush in 2004 and as we’re seeing with the House Republicans now, when they win elections, that’s what they try to do. But that’s not likely to be popular with seniors, and given the ground Republicans have lost among young voters and Latinos, that puts them in a tight electoral spot.

The White House knows it. Much of the president’s speech on Wednesday was devoted to a stirring defense of Medicare and Social Security. He spoke of the American conviction that “each one of us deserves some basic measure of security and dignity,” which is why “we contribute to programs like Medicare and Social Security.” He warned that the Republican plan “says that 10 years from now, if you’re a 65-year-old who’s eligible for Medicare, you should have to pay nearly $6,400 more than you would today.” Then he lowered the boom: “Put simply, it ends Medicare as we know it.”

Sounds like things are getting back to normal. But can a Republican Party reliant on the over-65 vote survive normal?

Ezra Klein blogs about domestic and economic policy for The Washington Post at washingtonpost.com/ezraklein.