At a meeting with top House and Senate leaders set for Thursday morning, Obama plans to argue that a rare consensus has emerged about the size and scope of the nation’s budget problems and that policymakers should seize the moment to take dramatic action.

As part of his pitch, Obama is proposing significant reductions in Medicare spending and for the first time is offering to tackle the rising cost of Social Security, according to people in both parties with knowledge of the proposal. The move marks a major shift for the White House and could present a direct challenge to Democratic lawmakers who have vowed to protect health and retirement benefits from the assault on government spending.

“Obviously, there will be some Democrats who don’t believe we need to do entitlement reform. But there seems to be some hunger to do something of some significance,” said a Democratic official familiar with the administration’s thinking. “These moments come along at most once a decade. And it would be a real mistake if we let it pass us by.”…

The administration argues that lawmakers would also get an important victory to sell to voters in 2012. “The fiscal good has to outweigh the pain,” said a Democratic official familiar with the discussions.

It is not clear whether that argument can prevail on Capitol Hill.