"The center of American politics gets older," said E. Mark Braden, a former chief counsel to the Republican National Committee who now advises elected officials and state legislatures. "Given the current fiscal concerns, it's going to be a test case whether Republicans or Democrats can talk about entitlement reform without getting killed" politically.
There now are 119 million people 45 and older, a group that makes up 51 percent of the voting-age population - up from 46 percent in 2000 and 42 percent in 1990. Those 55 and older represent the bulk of this group.
The preliminary figures are based on the Census Bureau's 2009 population estimates as well as the 2009 American Community Survey, which samples three million U.S. households. The 2010 census surveyed the entire nation.
Broken down into subgroups, older boomers ages 55 to 64 were the fastest-growing group since 2000, jumping 43 percent to approximately 35 million. They were followed by seniors 85 and older, who increased 33 percent to 5.5 million, due largely to medical advances that have increased life spans.
The number of people ages 45 to 54 also rose sharply, up 18 percent to 45 million.
Based on actual election turnout, which is higher for older Americans, census data show that those 45 and older represent 60 percent of voters in national races, judging by the 2008 presidential race.
"Boomers have now crossed the line between thinking about Medicare and Social Security as an issue for their parents, to being worried about it for themselves," said William H. Frey, a demographer at the Brookings Institution who did a broad analysis of available census data.
The Republican-controlled House last month approved a proposal to replace Medicare with payments to individuals to shop for private insurance. The measure would affect only those now under 55; people 55 and older would continue to be covered by Medicare.
The GOP plan bets that older Americans will accept Medicare changes if they are not personally affected.
So far, however, the heaviest resistance has come from older people, who oppose cuts that will affect their children and grandchildren or that conceivably could be expanded later to include them.