The heads of the family that owns the Chicago Cubs, Joe and Marlene Ricketts, gave Walker's Unintimidated PAC $5 million, and Wisconsin roofing magnate Diane Hendricks gave the PAC another $5 million.
The amount is a large number and a new reality in the era of super PACs -- but also shows how much candidates and their supporters must lean on a small number of ultra-wealthy donors.
The Ricketts family owns the Cubs and is set to be one of the most important donors for Walker as he competes with major fundraising operations like the one backing Bush. Joe Ricketts is the patriarch of the Ricketts family and a billionaire investor. His son, Todd, is Walker's campaign finance director.
Walker's super PAC -- which, like other super PACs, can collect unlimited amounts of money but cannot coordinate with the formal campaign -- filed its first fundraising report later Friday.
Walker was a late entrant into the race and has not been actively courting fundraisers as long as his competitors have been.
"Unintimidated PAC is drawing support from across the country because we stand behind a candidate with a proven record of fighting for conservative reforms and winning those fights in impressive fashion as a governor in a blue state," PAC spokesman Brad Dayspring said.
"Some organizations and groups are forced to rely on a handful of donors for nearly all of their resources, which is why we are proud to have a deep bench of enthusiastic supporters who are passionate about Governor Walker's candidacy and committed to our effort for the long haul," Dayspring continued.
Two other major donors helped lift the Walker team's first major fundraising effort: Richard Uihlein, a Wisconsin businessman and conservative megadonor, gave the PAC a combined $1.75 million. And another $1 million came via Access Industries in New York, whose founder, Len Blavatnik, is a major Republican donor.
Relying on big donations
Big donations from a small group could be a sign of weakness if that base doesn't expand, a problem faced by every candidate in the field not named Bush. According to a CNN breakdown of campaign spending,
most candidates' outside groups have relied on a small number of wealthy donors for ammo.
A pair of Texas oil billionaires
, the family of Robert Mercer and Toby Neugebauer, all combined to give Texas Sen. Ted Cruz rocketing into second place in the money race, giving him $36 million out of $38 million raised buy his outside groups.
And Florida Sen. Marco Rubio, who came in third place in the money race, had three megadonors contribute $11 million of the $16 million raised by Conservative Solutions PAC.
Bush's team, on the other hand, has attracted dozens of wealthy donors to his super PAC
-- a strong network that can play nationally and for the long haul.