Republican presidential candidate Sen. Ted Cruz speaks during the Freedom Summit on May 9, 2015 in Greenville, South Carolina.National Journal

Ted Cruz was a fierce defender of Trade Promotion Authority. Now, he is fighting against it.

In a blistering op-ed in Breitbart, Cruz announced Tuesday he no longer plans to support Trade Promotion Authority, legislation he voted for weeks that gives Congress an up-or-down vote on trade bills the president negotiates over the next six years. Citing concerns over immigration and "corrupt Washington backroom deal-making," Cruz reversed his position just hours before the Senate was poised to hold a procedural vote on the bill.

"Why does Republican Leadership always give in to the Democrats? Why does Leadership always disregard the promises made to the conservative grassroots?" Cruz wrote.

In order to back the bill, Cruz demanded he get immigration amendments that would block changes in immigrant flows from being included in any future trade bill as well as an explicit promise from Republican leaders that they would let the controversial Export-Import Bank expire at the end of the month. The unrelated Ex-Im Bank issue has become a rallying call for outside groups like Heritage Action, which scored against the House trade vote partly because leaders would not assure conservatives that the bank would cease.

"There's too much corporate welfare, too much cronyism and corrupt deal-making, by the Washington cartel," Cruz said in the op-ed. "For too long, career politicians in both parties have supported government of the lobbyist, by the lobbyist, and for the lobbyist — at the expense of the taxpayers."

In the op-ed, Cruz explains that he asked Majority Leader Mitch McConnell if the leader had secured a deal to reauthorize Ex-Im and was told that was not the case. However, Cruz was taken aback when Boehner would not promise conservatives that Ex-Im would expire. In a Twitter exchange Tuesday morning, Cruz spokesman Brian Phillips had strong words for those who were critical that Cruz had ever supported TPA. "Seems like you're asking why Cruz would believe anything the Majority Leader tells him? Fair point, I guess," Phillips wrote.

His reversal is significant as it is a departure from the presidential contender's previous statements on trade and marks a major blow for Republican leaders who are seeking to hold conservative support together for the trade bill. Initially, Cruz had been a key ally for GOP leaders. The freshman Texas senator had penned an op-ed with the bill's architect Paul Ryan in The Wall Street Journal praising the bill.

"The stakes are high, because if you're not moving forward in trade negotiations, you're falling behind," Cruz and Ryan wrote.

It appears now that the stakes are too high for Cruz to buck Heritage Action and vote for the trade bill.

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