"Goodness gracious," Obama said as he took the stage and peered at the human mass in Civic Center Park. People packed in all the way up the steps of the Capitol, off in the distance.
The Obama campaign released an initial crowd estimate of 75,000 people. That was later upgraded to "well over" 100,00 people, a tally confirmed by a Denver police spokesman.
The setting, on a sparkling day in this battleground state, said perhaps more than Obama did in his actual speech. His campaign is capitalizing on the scope of such rallies to get people to cast votes early, permitted in Colorado and more than two dozen other states.
"How many people have early voted?" Obama said, eliciting cheers from people bundled up in fleece. "That's what I'm talking about. No point in waiting in lines if you don't have to. You know who you're going to vote for."
Polls put Obama ahead in Colorado with the number of campaign days remaining now down to single digits.
It was here in Denver that Obama, in his groundbreaking campaign, accepted the Democratic Party's presidential nomination at the stadium where the Denver Broncos play. If elected, he would be the first black president of the United States.
"Do you ever have small crowds in Denver?" a smiling Obama said. Members of the crowd interrupted Obama's standard campaign speech with shouts of "Obama!" and "Yes we can!"
Traditionally, Colorado has gone for Republicans in presidential races, including twice for George W. Bush. Obama is trying to snag a win here as part of a multi-route path to capture at least the minimum 270 electoral votes on Nov. 4. ()
Colorado offers nine such votes.
Obama also jumped oncomment, made during an interview on NBC's "Meet the Press," that he and President Bush share a "common philosophy" of the Republican Party.
"I guess that was John McCain finally giving us a little straight talk, owning up to the fact that he and George Bush actually have a whole lot in common," Obama said.
"Well, here's the thing," he added. "We know what the Bush-McCain philosophy looks like. It's a philosophy that says we should give more and more to millionaires and billionaires and hope that it trickles down."
Obama, though, did not quote McCain fully.
The Republican presidential candidate also said: "I've stood up against my party, not just President Bush, but others; and I've got the scars to prove it." He also offered specific examples of differing with Bush, from Iraq strategy and deficit spending to campaign finance reform and climate change.
Obama was expected to speak before another large Colorado crowd later, in Fort Collins. McCain also has campaigned aggressively in the state, as has his running mate,.
More broadly, Obama is using his record-breaking fundraising advantage to buy up media time and make what he hopes is a closing argument for the presidency. McCain and his team say the race is hardly over, particularly for a candidate who's had his share of comebacks.
Obama released a new TV ad Sunday that describes McCain as Obama often does on the campaign trail - as "out of ideas, out of touch and running out of time." It also says McCain is resorting to smears and scare tactics because he doesn't have a plan to fix the economy.
The 30-second ad will begin running Monday on national cable television outlets.