GRESHAM, Ore. - Democrat Barack Obama told seniors yesterday that Republican John McCain would threaten the Social Security that they and millions like them depend on because he supports privatizing the program.

Later he held a spectacular, sun-splashed outdoor rally on the banks of the Willamette River in Portland. City fire officials estimated 65,000 people were packed into a riverside park, with 15,000 more left outside. Dozens of boaters listened from the river.

"Wow, wow, wow!" Obama said as he surveyed the audience. "We have had a lot of rallies. This is the most spectacular setting, the most spectacular crowd we have had this entire campaign."

His appearance before about 130 people at an assisted-living facility to talk Social Security was a subdued attempt to tie the GOP's presidential nominee-in-waiting to an unpopular President Bush on a pocketbook issue that motivates seniors - and also concerns younger generations worried about their own future retirement.

"Let me be clear: Privatizing Social Security was a bad idea when George W. Bush proposed it; it's a bad idea today," Obama said. "That's why I stood up against this plan in the Senate, and that's why I won't stand for it as president."

Bush proposed a Social Security plan in 2005 that focused on creating private accounts for younger workers, but it never came up for a vote in Congress. Democrats opposed the idea, and few Republicans embraced it.

Obama said McCain would push to raise the retirement age for collecting Social Security benefits or trim annual cost-of-living increases.

Obama has rejected both ideas as solutions to the funding crisis projected for Social Security in favor of making higher-income workers pay more into the system.

McCain spokesman Tucker Bounds accused Obama of making "misinformed partisan attacks."

"John McCain has been clear about his belief that we must fix Social Security for future generations and keep our promises to today's retirees, but raising taxes should not be the answer to every problem," Bound said.

While Obama was in Oregon, where he is favored to win the state's presidential primary tomorrow, his rival for the Democratic nomination, Hillary Rodham Clinton, spent a second straight day in Kentucky. She is favored to win there tomorrow.

She attended worship services at a Methodist church in Bowling Green and happily sang hymns and joined in Bible readings.

But her smile faded when the pastor launched into a sermon about adultery, asking his congregants whether the devil had ever whispered over their shoulders in their marriages.

Her mood appeared to brighten by the time she arrived for a rally at Western Kentucky University.

"Now, my opponent said the other day he wasn't coming back, so I've got the whole state to myself," Clinton said. "What a treat."

Obama, the front-runner for the Democratic nomination, has begun casting himself as the inevitable nominee and using his time to distinguish himself from McCain as he pivots toward the fall campaign.

He has scheduled appearances this week in Iowa and Florida, two key swing states.

He underscored that speaking with reporters in the Portland suburb of Milwaukie, saying he would use the Iowa visit as another way to focus on November.

"We thought it was a terrific way to kind of bring things full circle," Obama said.

"We still have some contests left, but if Kentucky and Oregon go as we hope, then we think we will have a majority of pledged delegates at that point, and that's a pretty significant mark. That means that after contests in every state, or almost every state and the territories, that we have received a majority of the delegates that are assigned by voters," he said.