WASHINGTON (AP) -- Republican frontrunner Mitt Romney and his party raised a sizable $40 million last month from donors who want him to replace President Barack Obama. But even as Romney solidifies his position as the eventual GOP nominee, many supporters who backed his primary election challengers have not yet come to his aid.

An Associated Press review of campaign finance data found that only a few hundred donors who contributed to candidates like Newt Gingrich or Rick Santorum have changed course and gave to Romney's campaign or the Republican Party in April. That's as GOP stalwarts and some former rivals have called on supporters to rally around Romney's White House run.

Romney is hardly hurting for cash, having reported more than $61 million in the bank by the end of April. But financial reports released Sunday reveal a potential struggle for Romney in persuading his party's more conservative donors to open their wallets for him, although there are still more than five months until Election Day.

Out of more than 50,000 donors who gave to other GOP candidates like Gingrich, Santorum and Texas Gov. Rick Perry since the start of the nomination race, fewer than 600 appeared to write checks to Romney in April, the AP's review of Federal Election Commission reports found. Roughly the same number contributed to the Republican National Committee, which is now helping Romney's campaign.

Romney has been aggressively courting conservatives, speaking out recently against China's "one-child policy" and addressing graduates at the evangelical Liberty University. Last month, Romney told the website Breitbart.com that the news media were involved in a "vast left-wing conspiracy to work together to put out their message and to attack me."

Wealthy donors like former Santorum supporter Foster Friess are now supporting Romney, realizing that his sizable delegate count and financial strength all but guarantee his nomination this summer. Other eventual Romney donors had already given early to his rivals, partly to support a spirited debate during the primary season.

Willis J. Johnson, founder of auto dealer Copart USA, contributed to Perry and Gingrich last year because he personally liked Perry and admired Gingrich's insistence on quizzing Romney during debates. "I think this administration is trying to take money away from small businesses," Johnson said, in part through tax hikes.

Other conservatives, however, have been warier. One blog post last week on the conservative RedState.com, referring to Romney's Liberty speech, asked: "Mr. Romney said that marriage is between one man and one woman. He got deafening applause. But where was he when it mattered?"

About a quarter of Romney April-turnaround donors gave checks of $2,500 or more, including amounts up to the maximum $30,800 legally allowed to a political party. In March — just before Romney started raising general election cash — about 300 former Santorum and Gingrich supporters contributed to the former Massachusetts governor's campaign.

Santorum dropped out of the race in April and Gingrich in May, even though Romney's momentum was growing before then. A report detailing the contributions of Romney's joint-fundraising committee is due out this summer.

A Romney spokeswoman did not immediately respond to requests seeking comment late Sunday.

The AP analyzed federal contributions since January 2011, cross-matching donors' names and addresses with those on Romney's or the GOP's April reports. Those filings, per federal election requirements, exclude donors who gave fewer than $200 per candidate each election — although most of Romney's campaign contributions ($8.9 million) came from donors who gave $200 or more.

April's financial reports, due to the FEC by midnight Sunday, showed that Obama and the Democratic Party brought in a combined $43.6 million last month. Obama's campaign has had to raise funds aggressively to answer ads from conservative "super" political committees.

Indeed, Romney's fundraising bounty doesn't include the hundreds of millions of dollars flowing into GOP super PACs. On Sunday, the Romney-supportive Restore Our Future said it raised $4.6 million last month, leaving the group with $8.2 million cash on hand.

Super PACs can raise and spend unlimited amounts of cash but can't coordinate with the candidates they support. GOP groups have largely outraised their Democratic counterparts, although Obama has a strong financial position that traditionally comes with being an incumbent.

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