JACKSON, Mississippi — Hours before two major Tea Party rallies in this state, the American Conservative Union endorsed state Senator Chris McDaniel over incumbent Sen. Thad Cochran, providing the imprimatur of an organization that remains the most highly-sought validator of a conservative voting record in Congress.

The group, which, despite being conservative has strong ties to the GOP establishment, has emerged as a surprising thorn in Cochran’s side, issuing him an extremely low rank on its 2013 voting scorecard as well as during the Obama era.

McDaniel, who has tapped into enthusiasm from the grassroots but faces the powerful political machine of former Gov. Haley Barbour in taking on Cochran, touted the endorsement at the second of two stops Thursday.

“The American Conservative Union which is the preeminent grader–one of our oldest and strongest proponents of conservative values in the country–they refused to endorse Thad Cochran,” McDaniel said at the Jackson Tea Party Express bus tour rally.

“In fact they graded him along with the other Republican senators during the Obama presidency and as it turns out, during the Obama presidency, Thad Cochran received a D- from that organization. Only one other senator was worse on the Republican side of the aisle than Sen. Cochran: Susan Collins of Maine. This is not Maine. This is Mississippi. We have to learn to fight like Mississippians if we’re going to reclaim this country so this morning I’m pleased to announce the American Conservative Union did endorse somebody this morning. They endorsed us this morning,” McDaniel added.

The ACU’s dismal rating for Cochran’s 2013 voting record – he scored 63 percent – surprised many political insiders because of the group’s relatively strong ties to the Establishment.

The endorsement pits ACU chairman Al Cardenas, Norquist, former House Majority Leader Tom DeLay, ex-Hewlett Packard CEO Carly Fiorina, and others against Barbour, now a lobbyist and Cochran’s biggest supporter. Barbour himself is backing Cochran, and his two nephews are each running a part of Cochran’s campaign: Austin Barbour is a senior Cochran campaign strategist and Henry Barbour is running the pro-Cochran Super PAC “Mississippi Conservatives PAC.”

Cocrhan’s campaign declined to comment on the scathing assessment from ACU.

At the rally here in Jackson, McDaniel told about 80 supporters “We all know intuitively that Washington, D.C., is broken,” McDaniel said in his remarks during the Tea Party Express bus tour’s second Mississippi stop on Thursday. “We know the country is going the wrong direction. And it begs the question, if it’s going the wrong direction–we can’t change Washington, D.C., until we change the people we send there to represent us.”

“I still respect Thad Cochran,” McDaniel said. “I like Thad Cochran. But when he first went there in 1973, Richard Nixon was your president. It was a different time then: perhaps at the time it was fashionable to spend money recklessly but those days have passed.”

McDaniel invoked the mantle of a generation of conservative leaders in the Senate who have changed the dynamic of the traditionally staid chamber.

“This is a new era, and we have to fight with courageousness,” McDaniel said. “We have people in the U.S. Senate that have given us a message of courage. Ted Cruz is one of those men. Mike Lee is one of those men. Rand Paul is one of those men. But I’ll tell you this, from the heart of Mississippi, we should not have to depend on Texas to fight our fights for us. We should not have to depend on Kentucky to lead our fights for us, or Utah to fight our fights for us. We’re the most conservative state in the Republic and it’s time we led that way.”