COLORADO SPRINGS, Colo. - His cash advantage threatened, President Barack Obama and his party are redoubling their fundraising efforts after robust hauls by Republican rival Mitt Romney and a slew of GOP-leaning super PACs that are raking in cash from the party faithful highly motivated to topple the Democrat.

Obama still has a significant edge, but it's shrinking rapidly.

That explains why the president, fresh off back-to-back international summits, was plunging back into his re-election race Wednesday with a series of fundraising events in Denver and California's Silicon Valley as he looks to stockpile cash to pay for his coast-to-coast organization, advertising to spread his message and get-out-the-vote operations in key states.

It's the start of an extensive money push by Obama in the coming weeks that will feature a series of high-end fundraisers, including New York events with former President Bill Clinton and actress Sarah Jessica Parker and a Los Angeles trip to raise money among gay and lesbian supporters. Smaller-dollar pushes also are under way.

Obama, a record-shattering fundraiser four years ago, has a built-in fundraising advantage as the incumbent and still has a wide money lead over Romney, the challenger who only recently combined fundraising efforts with the Republican National Committee after a bruising - and expensive - primary.

But well-funded Republican outside groups, which are able to raise unlimited sums from donors, are narrowing that gap quickly and using their multimillions to run a slew of TV ads hammering Obama in key states. Obama aides acknowledge the possibility that he could be outraised by the influx of Republican money.

The numbers tell the story.

Through April, Obama and Democratic groups supporting his re-election bid have raised nearly $450 million during the election cycle and have more than $150 million in the bank. Romney and Republicans backing him have collected more than $400 million during the same stretch and have about $80 million at their disposal.

Gone is the 10-to-1 cash advantage that Obama held at the end of March.

To be sure, Romney was bound to erode that money gap as he pivoted to the general election. He still, however, lags on another measure of campaign strength: Obama has had months to prepare an extensive ground game to identify, register and turn out voters.

On the money front, Romney has benefited from a strong desire among GOP activists to defeat Obama, multiple GOP outside groups willing to spending tens of millions of dollars and a well-oiled fundraising machine within his own campaign. Showing that prowess, the former Massachusetts governor raised $15 million this week during three days of fundraising in New York.

"What you see in a very short period of time is a very well-run operation that is putting Gov. Romney in a position where he's going to, maybe not outspend, but to compete with the collective Democratic fundraising," said Sara Taylor Fagen, a former political director for President George W. Bush.

Romney has been the all-but-certain GOP nominee for more than a month now, and while he's focused primarily on fundraising, super PACs backing him have been going toe to toe with Obama's campaign in TV advertising. That means that Romney hasn't had to spend heavily from his own campaign account. Chief among those groups has been Crossroads GPS and its affiliated super PAC, Crossroads USA, which quickly matched Obama's ad buy this month after the president's team laid out plans for a $25 million advertising campaign.

Democrats haven't had as much success with super PACs.

A pro-Obama group, Priorities USA Action, has badly lagged behind Crossroads, while Romney has gotten extra help from another super PAC, Restore Our Future. Mary Beth Cahill, a former campaign manager for Sen. John Kerry, recently came aboard as an adviser to help.

The influx of campaign cash in the first presidential campaign since the 2010 Citizens United ruling by the Supreme Court, which helped create super PACs, has taken some Democrats by surprise.

"I don't think anyone realized going into this cycle exactly how much money was going to be involved," said former Rep. Martin Frost, D-Texas, a past chairman of the fundraising arm for House Democrats. "This is a brave new world of campaign finance."

The president was headed Wednesday to fundraisers in Denver and California's Silicon Valley. He was addressing about 700 supporters in Denver, with tickets starting at $250 and topping out at $40,000 per couple for a photo with the president.

Afterward in California, Obama was speaking at a fundraising dinner in Atherton that includes a performance by David Crosby and Graham Nash. About 60 people were paying $35,800 per person to attend.

Obama was capping the night at a reception with 1,100 people in Redwood City, with a performance by Ben Harper. Tickets started at $250, with some couples paying $12,500 for a photo with the president. On Thursday, Obama was speaking at a private fundraising breakfast in Palo Alto.

To keep his edge, Obama isn't just focusing on big money.

Many of the planned high-dollar fundraisers include a raffle designed to raise millions more and get more people involved.

In some cases, the grass-roots component raises more than the swanky fundraiser: Of the $15 million Obama raised at a celebrity-studded dinner two weeks ago at actor George Clooney's Los Angeles home, $9 million came from small-dollar donors hoping to win a chance to attend.

Clinton, arguably the most prominent Democratic fundraiser not in the White House, is joining Obama for two events in New York on June 4. Obama's campaign also is raffling off a trip to New York - including airfare and hotel for what's being called "Barack on Broadway" - for two winners and their guests to attend. The two presidents will attend a dinner later that evening featuring a performance by Jon Bon Jovi.

Two days later, Obama jets to Los Angeles for a high-dollar reception with gay and lesbian supporters, featuring a performance by Pink, and a $25,000-per person dinner at the Beverly Hills home of "Glee" creator Ryan Murphy and his fiance David Miller. Events also are planned next month in Baltimore, Boston and back in New York, where the president will raise money at the home of Parker, of "Sex and the City" fame. A travel-package raffle for small-donors is tied to that, too.

"It should be fabulous," Parker said in an email to Obama supporters.

Obama began airing two new ads Wednesday, one about his work with veterans returning from wars in Iraq and Afghanistan and another aimed at seniors dependent on Medicare.

In the veterans spot, Obama credits veterans for allowing the U.S. to "go after al-Qaida and kill (Osama) bin Laden" and says the nation has a "sacred trust" to help veterans heal their wounds and find jobs. The ad on Medicare notes that Obama was raised by his grandparents and cites his administration's efforts to root out health care fraud.

The TV ads are part of the $25 million ad buy the Obama campaign launched in May, running in Colorado, Florida, North Carolina, Iowa, Nevada, Ohio and Virginia.

Romney spokeswoman Amanda Henneberg said Obama's team can't "distract voters from three years of broken promises on Medicare and our commitments to our veterans."