In the fundraising mailer, Cruz urges donors to "pull out all the stops" in order to help elect "solid, principled, conservative fighters."
The Madison Project, albeit not as well known as bigger conservative outside groups like the Senate Conservatives Fund and FreedomWorks, has endorsed and boosted a number of conservative primary challengers to incumbent Republican lawmakers including Republican Matt Bevin in Kentucky who is running against Senate Minority Leader Mitch McConnell (R-KY) and Milton Wolf who is challenging Sen. Pat Roberts (R-KS).
The tea party icon has promised to publicly stay out of even the most divided Republican primaries.
Cruz spokesman Catherine Frazier told Politico the letter was sent out "based on a previous fundraising agreement last year."
In a fundraising email on Wednesday The Madison Project's Drew Ryun wrote that Politico was doing a hit piece on his organization.
"It is clear the long knives of the GOP Establishment are out for the Madison Project, the Senate Conservatives Fund, Ted Cruz and Mike Lee. You know what that tells me? We are a serious threat to them," Ryun wrote. "Last night the Politico published what was clearly intended to be a hit piece on us."
This story was updated.