Donald Trump buttons are displayed at a rally for the candidate, in Dubuque, Iowa, Jan. 29, 2016. Scott Olson/Getty Images

Republican presidential candidate Donald Trump has estimated that his campaign will need $1.5 billion to compete against Hillary Clinton. Now the finance chairman of the Republican Governors Association is predicting that Trump could fall well short of his goal.

"Unless he's willing to write a huge personal check, which is unlikely, I believe Trump will have a financial disparity of $300 million to $500 million," RGA Finance Chair Fred Malek told the Wall Street Journal in a story published on Tuesday evening. In other words, the billionaire businessman may make it only two-thirds of the way to his fundraising target.

Malek's prediction reflects the reality that, while the Republican Party has tentatively lined up behind its presumptive nominee, the money hasn't followed. Billionaire activists Charles and David Koch — the brothers who together lead the most powerful conservative donor network in the country — have withheld their support from Trump, and seem likely to concentrate more of their energies down-ticket. Some Koch associates, like casino mogul Sheldon Adelson, have gotten behind Trump, but the conservative megadonor class is far from unified.

Meanwhile, some of the prominent Republicans who could help steer money into Trump's coffers are instead keeping their distance. Wisconsin Gov. Scott Walker, a failed 2016 Republican candidate and a favorite of the Koch network, has shied away from endorsing his party's choice. House Speaker Paul Ryan — another Koch favorite — has offered Trump a lukewarm endorsement at best while also denouncing some of his public remarks as "racist."

Trump probably shouldn't hold his breath waiting for either Ryan or Walker to headline a fundraiser on his behalf.

Other members of the party machinery are working to build fundraising infrastructure for Trump. The joint fundraising committee known as the Trump Victory Fund, launched in late May, is already raising millions, though it still lags far behind the Democratic Party's Hillary Victory Fund. According to data collected by the Sunlight Foundation, the Trump Victory Fund appears to have held three fundraisers since its launch, with a fourth to take place in Richmond, Virginia, on Friday; Clinton's own joint fundraising committee has existed since last year and has hosted hundreds of fundraisers.

As the Sunlight Foundation reported, federal law allows a single donor to contribute as much as $449,400 to the Trump Victory Fund. But only $5,400 may be earmarked for the Trump campaign itself, with the rest going toward other Republican entities like the national committee and the state parties.

Donations in $5,400 increments will get Trump only so far, particularly when federal disclosure forms from the end of April reported he had just a little more than $2.4 million on hand. Hillary Clinton had more than $30 million.