Editor's note: This story has been updated throughout.

At a rally in Indianapolis Wednesday, Ted Cruz named former presidential rival Carly Fiorina as his would-be running mate if he can manage to snag the Republican presidential nomination from frontrunner Donald Trump.

"After a great deal of time and thought, after a great deal of consideration and prayer, I have come to the conclusion that if I am nominated to be president of the United States, that I will run on a ticket with my vice presidential nominee, Carly Fiorina," Cruz said.

In laying out what he was looking for in a vice-president, much of Cruz's comments served as an implicit criticism of Trump. 

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"Do they pop off the handle at whatever strikes them at any given moment?" he asked, and then offered Fiorina as a contrast, complimenting her judgment and character

He also praised her Texas roots, beginning his biographical description of her with "Born in Texas...the very first thing I liked about her."

Fiorina followed up with her own commitment to Cruz and the campaign.

“This is the fight of our time, and I believe Ted Cruz is the man to lead that fight," Fiorina said. "And I am prepared to stand by his side, and give this everything I have to restore the soul of our party, to defeat Donald Trump, to defeat Hillary Clinton and to take our country back." 

Cruz's campaign had announced earlier this week that it had come up with a short list of potential candidates for vice president and that it was vetting Fiorina, the former Hewlett Packard CEO.

Reports swirled Wednesday ahead of the rally. New Hampshire-based ABC affiliate WMUR announced Cruz would pick Fiorina as his running mate, as did Politico. Campaign sources confirmed the selection to the Tribune shortly before the rally.

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Some speculation had centered on Indiana Gov. Mike Pence as a possible running mate, but Pence's office said Wednesday that the governor is not scheduled to attend the Indianapolis rally.

Read MoreThe Texas Tribune's 2016 Presidential Election Coverage

Cruz's announcement comes a day after Cruz suffered a series of crushing defeats to frontrunner Donald Trump in five northeastern primaries. Declaring himself the presumptive nominee Tuesday night, Trump said Cruz would be "wasting his time" by naming a running mate. 

The announcement is highly unconventional in presidential politics. It also immediately proved controversial.

Historically, major candidates refrain from naming a running mate until the lead up to the party convention. Republicans will host their national convention in Cleveland in July.

Moreover, Cruz is not the presumptive GOP nominee. Trump has a 392-delegate lead, per an NBC News delegate count

But over the course of the campaign, Cruz and Fiorina struck up a chemistry on the campaign trail that was reminiscent of the relationship between 2012 nominee Mitt Romney and his eventual vice presidential pick, U.S. Rep. Paul Ryan of Wisconsin, who is now the speaker of the U.S. House of Representatives. 

A Cruz insider told the Tribune that the pair coordinated well together and Fiorina impressed the Texas senator with "the fiery and firm manner in which she campaigns."

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After dropping out of the race in February, Fiorina endorsed Cruz a month later. She has since campaigned with him about a dozen times and completed her own solo campaign swings on Cruz's behalf. At one of their joint campaign appearances in March, Fox News host Sean Hannity posed the question of who Cruz would want as a vice president.

"I'll say this," Cruz said. "Carly is extraordinary." 

The Fiorina pick does two immediate things for Cruz. Most immediately, it helps change the subject from the blowout Trump dealt him Tuesday night in five East Coast states: Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island. (Cruz took a similar tack after a disappointing finish in the Nevada caucuses on Feb. 23: The next day, his campaign announced the endorsement of Texas Gov. Greg Abbott.)

Additionally, Fiorina's appearances on the campaign trail will likely now pick up more attention and enthusiasm, helping renew the Cruz's campaign's flagging momentum.

Democrats took the news as a cause for celebration, gleefully pointing to Fiorina's stumbles as CEO of Hewlett-Packard as easy pickings if Cruz were to outmaneuver Trump for the GOP nomination.

"This is a desperate move from a desperate and dying campaign," Democratic National Committee CEO Amy Dacey said in a statement. "It’s fitting that a man best known for shutting down the U.S. government would select a woman best known for almost shutting down a great U.S. company." 

Within Cruz's inner circle, Fiorina has been in consideration as a vice presidential contender for several weeks, if not further back. Yet multiple Republican operatives described the decision to make the announcement on Wednesday as a "Hail Mary."

Essentially, Cruz's only path to the nomination is to prevent Trump from winning a majority of delegates and to force a floor fight at the Cleveland GOP convention. To accomplish that, his political fortunes mostly rest in two coming state primaries: Indiana on Tuesday and California on June 7.

Indiana is an absolute must-win for Cruz when it comes to regaining momentum and drawing potential delegates away from Trump. Cruz hinted at those high stakes Wednesday morning in Indianapolis, hours before his announcement.

"Last night was Donald Trump's night," Cruz told reporters, "and today is Indiana's day."

Campaigning Wednesday morning in Indianapolis, Cruz told reporters he would make a major announcement at a rally in the afternoon. While he declined at the time to say whether he planned to name a running mate at the rally, Cruz reiterated his criteria for such a selection. 

"As I have said from the beginning, the most important attribute for any running mate is that he or she should be able to step in and fulfill the role as president, be the commander in chief, keep this country safe, and champion jobs, freedom and security," Cruz said.  

Vincent Harris, a Texas-based GOP digital strategist who worked for Cruz's 2012 Senate campaign and later U.S. Sen. Rand Paul's failed presidential bid, wrote a series of highly critical tweets of Cruz's handling of the announcement Wednesday. Specifically, he argued that the way the story leaked prevented Cruz's team from turning the traditional intrigue around the running mate announcement into a source of data collection for the party.

"Hail Mary had to happen but it stinks from a digital perspective that there isn't full digital support from the party to maximize," Harris wrote

Referring to the 2008 presidential candidates and their running mate picks, Harris added, "Obama's famous mobile program? Built off of Biden's VP announcement..McCain's digital fundraising? Built off of Palin's announcement."

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