WASHINGTON – For the first time, Americans 45 and older make up a majority of the voting-age population, giving older Americans wider influence in elections as the U.S. stands divided over curtailing Medicare and other benefits for seniors.

Along with the information about the growing influence of older adults, preliminary census estimates also show a decline in the number of married couples with children, slight growth in household size and a rapid rise in the number of Mexicans.

The findings, based on the latest publicly available government data, offer a preview of trends that will be detailed in the next round of 2010 census results being released this month that focus on age, household relationships and racial subgroups.

As a whole, the numbers point to a rapidly graying nation driven largely by the nation’s 78 million baby boomers, who are now between the ages of 46 and 65 and looking ahead to retirement.

“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.

Currently, there are roughly 119 million people 45 and older who make up 51 percent of the voting-age population, with Americans 55 and older representing a large bulk of that group. The new majority share is up from 46 percent in 2000 and 42 percent in 1990.

The preliminary figures are based on the Census Bureau’s 2009 population estimates as well as the 2009 American Community Survey, which samples 3 million U.S. households. The 2010 census surveyed the entire nation.

Broken down by subgroups, older boomers ages 55-64 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 more than 5.5 million, due largely to medical advances that have increased life spans.

The number of people ages 45-54 also rose sharply, up 18 percent to 45 million as young boomers moved into the ranks.

Based on actual election turnout, which is higher for older Americans, census data show that baby boomers and seniors ages 45 and older represent about 60 percent of voters in national races, judging by the 2008 presidential race. Nearly 1 out of 2 voters is 50 or older.