Washington ? Democrats have glimpsed their biggest threat this fall, and she is Grandma.

One reason President Barack Obama and other party leaders are rolling out campaigns to energize young and minority voters for November’s elections is that they’ve seen the polling data on senior citizens, and it’s ugly.

Democrats once counted on voters older than 65, but many seniors’ loyalties changed in the past decade. That’s partly due to the parties’ stands on policy such as fiscal and personal responsibility, and partly, experts said, because the demographic changed. Many of today’s seniors started voting in the 1950s, when Republican Dwight Eisenhower was president. The number of people whom FDR converted to Democrats in the ’30s is shrinking fast.

A majority of seniors backed Republican Sen. John McCain of Arizona over Obama in 2008. The seniors’ current tilt toward Republicans is being reinforced by Obama’s biggest accomplishment as president: the health care overhaul.

A George Washington University Battleground Poll in April found that only 36 percent of likely voters who were 65 and older approved of the health care law, which expands and mandates health insurance coverage. That was the lowest approval rating of any voter age group.

For November’s congressional elections, the poll found that seniors are far more inclined to vote for Republicans than for Democrats, by 48 percent to 33 percent.

“Other than enthusiasm of our base, it’s probably the biggest problem we face,” said Celinda Lake, the Democratic pollster partner of the bipartisan poll.

Seniors’ view of the health overhaul could change before the elections, of course. Several insurers have announced in recent days that they’ll speed up their participation in some consumer-friendly elements of the law. Further, California Gov. Arnold Schwarzenegger, a Republican, on Thursday announced his plans to put politics aside in the interest of making the overhaul work for residents of his state, a move that Health and Human Services Secretary Kathleen Sebelius praised.

Seniors are far more likely to vote in nonpresidential elections than young voters are. In the latest Gallup tracking poll, 41 percent of seniors were “very enthusiastic” about voting this year, nearly twice as high as registered voters younger than 30.

The health care debate alarmed a lot of seniors, said Robert Binstock, a professor at Cleveland’s Case Western Reserve University who focuses on aging and health.

“This whole business about death panels and pulling the plug on granny has whipped up older people,” Binstock said.

Even though many Republican myths about the health care law have been debunked, Binstock said seniors remained worried about the law’s impact on Medicare and the care they received.

Seniors so far haven’t been swayed by Democrats’ assurances that the law will extend Medicare’s solvency by a decade or longer, reduce the budget deficit and cover the Medicare Part D prescription-drug “doughnut hole” for seniors within a decade, starting with $250 rebates this year.