Michael Winerip reports:

In a Pew Research Center poll of likely voters, Mr. Obama is ahead, 50 to 44 percent, within [the baby boomer] age group.

“This is a significant change, and that generation’s vote counts for a lot,” said Scott Keeter, director of survey research for Pew. The boomers, who range in age from 48 to 66, account for 37 percent of all voters, according to Pew. Generation X — roughly 31 to 46 — makes up 26 percent. The youngest voters, ages 18 to 30, and the oldest, over 65, make up 17 percent each.

The apparent shift could be offset by a softer level of support for Mr. Obama among the youngest voters, in the 18-to-29 age group, of whom 66 percent went for him in 2008, and potentially lower turnout.

What is moving the baby boom voters? It may be Medicare.

Read more from the Pew Research Center’s latest report on voter preferences and the campaign.