According to a New York Times report by Nicholas Confessore:

On Monday, at a fund-raising event in Manhattan for the New York State Republican Party, David Koch told donors that he and his brother, who oversee one of the biggest private political organizations in the country, believed that Mr. Walker would be the Republican nominee.

“When the primaries are over and Scott Walker gets the nomination,” Mr. Koch told the crowd, the billionaire brothers would support him, according to a spokeswoman. (snip)

 Two people who attended the event said they heard Mr. Koch go even further, indicating that Mr. Walker should be the Republican nominee. A spokeswoman disputed that wording, saying that Mr. Koch had pledged to remain officially neutral during the primary campaign.

Assuming that David Koch speaks for his brother Charles, this support could be a deal-maker for Walker, because the Kochs not only generously give their own money, but also lead a network of organizations, such as Americans for Prosperity, that attract other substantial donors.  However, David Koch has issued a clarification (via Politico):

A Koch spokesperson sends the following statement from David Koch:

While I think Governor Walker is terrific, let me be clear, I am not endorsing or supporting any candidate for President at this point in time.

This would seem to mean that no huge wave of Koch money will flow into the Walker primary campaign immediately.

Some quick implications:

1. This has to be a blow to Rand Paul, as the Kochs have a strong libertarian background.  Such a signal from David Koch could cause other libertarian donors to think twice before committing to Senator Paul.

2. The left will take this as a signal to tar Walker as a puppet of the Koch brothers.  While there is no evidence that Harry Reid’s obsessive demonization of the brothers had any effect, if nothing else, it will spur the left to attack Walker even more than it has in the past.  And considering that Walker has taken on the government worker unions, amping up the fury of attacks would seem to be difficult.

3. It may increase media scrutiny of Walker at a time when he prefers to remain relatively quiet.  He is working now to build up his foreign policy expertise, as Dylan Byers of Politico notes:

Since mid-March, the once press-friendly Walker has been limiting his media availablity, closing off events to reporters and refusing to take questions. That pattern continued during Walker's recent visit to Europe, where he held no public events and took no questions from reporters, and is set to continue next month when he visits Israel. Last week, an aide told The Wall Street Journal that Walker "will focus on educating himself about Israeli issues and won’t hold public events or take questions from reporters."

4. It is already a cliché that a “secret primary” or “money primary” is underway, and until now, Jeb Bush has been the presumptive favorite among the donor class.  If Koch support flows toward Walker, Bush’s momentum could be further damaged.