Former Massachusetts Gov. Mitt Romney was poised to collect as much as $20 million during the fundraising quarter that ends Thursday, advisers said, as his rivals for the GOP presidential nomination struggled to keep pace with the campaign's front-runner.

With the three-month reporting period's end in sight, the crowded field of GOP contenders made final-hour pitches for contributions that will be an early measure of their campaign's strength _ or weakness.

Romney, scheduled for fundraisers in the Washington area on Wednesday and in Philadelphia on Thursday, was positioned to far outraise other Republicans looking to challenge President Barack Obama's well-financed re-election campaign for 2012.

Romney, however, is the candidate to beat now. He raised more than $10 million in a single day in Las Vegas and has worked the GOP donor community hard. On Wednesday, he was holding a $2,500-a-person fundraiser in a Washington suburb after meeting privately with senators on Capitol Hill to pitch himself as the candidate in the strongest position to make Obama a one-term president.

Romney is expected to report raising $16 million to $20 million for the quarter, said two advisers who spoke on the condition of anonymity to discuss fundraising before the deadline. Romney, who pumped $42 million of his own fortune into his 2008 presidential race, was not planning to write himself a check this time and instead has canvassed hotbeds of GOP donors such as Texas, California and New York.

Romney's rivals weren't expected to match those sums.

"Your investment and commitment will send a clear message to President Obama: Failure is not an option for our nation. Please make a secure online donation through my website," former Minnesota Gov. Tim Pawlenty, who is struggling to attract cash, said in a message to supporters.

"I wanted to reach out, as I just received another call from my finance director. She reminded me that our fundraising deadline is June 30 _ this Thursday _ and asked if I could reach out to our supporters nationwide," former Sen. Rick Santorum wrote to his backers.

"In the coming months, Mary Kaye, the children and I will be traveling across the country meeting with Americans just like you while talking about our plan to make America great again. But we will need your help," former Utah Gov. Jon Huntsman wrote to his supporters.

The campaigns have until July 15 to disclose their official hauls _ and expenses, which could be more than they raised for some candidates _ but advisers expected to release highlights in the coming days. Those numbers will help still-undecided donors pick which of the candidates to support.

"You've got to be a little bit crazy and a little intrepid to talk about putting together a presidential campaign that you'd like to see others put money into," Huntsman said during a recent interview.

Pawlenty, who for years has laid the groundwork for a presidential bid, has faced hurdles to raising funds, and some of his advisers are working without pay. Pawlenty aides say the fulltime staff continues to collect paychecks and the campaign will report raising enough money to finance their operations in early nominating states Iowa, New Hampshire and South Carolina.

Former House Speaker Newt Gingrich, who has amassed more than $1 million in campaign bills, was expected to report fundraising troubles that would leave him in the red. While running his network of not-for-profit, advocacy and consulting organizations, Gingrich was able to raise millions from a small web of donors. Now a presidential candidate, he is limited by federal election laws and has not invested the time needed to build his donor lists.

His senior campaign aides, including two of his fundraising chiefs, resigned en masse earlier this month over disagreements with Gingrich.

Santorum, who spent almost $26 million in his unsuccessful 2006 Senate re-election bid, was expected to trail in fundraising despite early efforts and frequent visits to the first states to have nominating contests. His previous race put him in the national spotlight as Republicans tried to defend the Pennsylvania Senate seat, but Santorum has been out of office since 2007 and donors' interest has waned.

Rep. Michele Bachmann, who formally began her campaign earlier this week, is a proven fundraiser; she raised more than $13 million for her re-election to her Minnesota district and could tap into those donors who see her as a chief spokeswoman for the tea party. Yet she hasn't been a serious candidate long enough to test her fundraising reach for a national race.

Rep. Ron Paul of Texas, a libertarian and tea party favorite, always draws impressive fundraising sums from his loyal followers but hasn't yet made national inroads on his third White House bid. And Herman Cain, the Georgia businessman who never has held elected office, could surprise with his results: An unexpected third-place position in this week's Des Moines Register poll of Iowa caucus participants gave him an unexpected buzz.

"It's not easy," said Huntsman, who raised $1.2 million during an event in New York last week. "And, typically, you don't get money from them on your visit."