Ted Cruz will announce Tuesday he is picking former rival Carly Fiorina as his running mate, NPR's Sarah McCammon confirms.

The "major announcement" Cruz has teased at 4 p.m. is an unprecedented, last-ditch move designed to shake up the GOP primary race, one in which he badly trails front-runner Donald Trump.

Fiorina, the former Hewlett-Packard chief executive, ran for president herself earlier this year before withdrawing after a disappointing showing in New Hampshire. She peaked early in the contest with a forceful showing in the first GOP debate, and proved herself to be a salient attack dog against Democrat Hillary Clinton. She endorsed Cruz in early March and has been a frequent, energetic surrogate for the Texas senator.

But actually getting the duo's ticket to victory in the Republican race is an uphill, and some would say improbable, battle. After Trump's Northeastern sweep on Tuesday night, it's mathematically impossible for Cruz to net enough delegates to win on the first ballot if Trump doesn't reach the requisite 1,237 delegates needed to clinch the nomination.

Cruz trails Trump now by almost 400 delegates, and he needs a big win next Tuesday in Indiana to even hope to close the gap. The surprise pre-emptive announcement of Fiorina as his running mate is designed to try and move more support in the Hoosier State his way. There were more machinations to try and salvage his hopes in Indiana over the weekend when he and Ohio John Kasich's campaign announced an informal alliance where the Ohio governor (who's in a distant third in delegates) agreed he wouldn't campaign in Indiana while Cruz ceded New Mexico and Oregon to Kasich.

Cruz has proven more adept than Trump in wooing unpledged delegates and winning over activists at state conventions, including some Trump-pledged delegates who could break for him at a multi-ballot convention.

His announcement of Fiorina is a hail mary political calculation designed to salvage his fading White House hopes. In 1976, Ronald Reagan tried something similar by announcing he would name Pennsylvania Sen. Richard Schweiker as his would-be vice president. But that move came after all primaries had concluded, and he was still trailing President Gerald Ford in delegates. The gamble failed and the incumbent Republican wrestled enough delegates to secure the nomination on the first ballot.

Fiorina could be helpful for Cruz in another state where he badly needs to stop Trump: California, which votes on the last major primary day on June 7th. She unsuccessfully ran for Senate in 2010 in the state. But her selection could also re-open criticisms that resurfaced during that failed run and her own White House campaign over her tenure and eventual ouster from HP coupled with layoffs at the tech company.

The Cruz announcement of Fiorina as his running mate designate comes after Clinton has also essentially clinched the Democratic nomination, which would make her the first female major party nominee in history. Republicans believed early on in the race that Fiorina, the only woman in the GOP race, could uniquely criticize Clinton in a way without the backlash that other male candidates might risk.

If Trump is the nominee, many Republicans worry he will alienate female voters in the general election given his past controversial comments on women — a fear exacerbated by his victory speech last night where he blasted Clinton for playing "the woman's card."

"If Hillary Clinton were a man, I don't think she would get 5 percent of the vote," he jabbed at the end of his remarks.

Trump has been critical of Fiorina as well in the past, particularly her looks.

"Look at that face!" Trump said in a Rolling Stone article last September. "Would anyone vote for that? Can you imagine that, the face of our next president."

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