INDIANAPOLIS -- In need of momentum after a five-state shutout, Republican presidential candidate Ted Cruz on Wednesday tapped former technology executive Carly Fiorina -- a woman who he said has repeatedly "shattered glass ceilings" -- to serve as his running mate.

The Texas senator announced his pick for vice-president at an Indianapolis rally, an unusual move for an underdog candidate that reflects the increasing urgency for the fiery conservative to reverse his downward trajectory.

Cruz praised Fiorina's path from secretary to CEO and her past willingness to challenge GOP front-runner Donald Trump.

"Carly isn't intimated by bullies," he declared, adding, "Over and over again, Carly has shattered glass ceilings."

The 61-year-old Fiorina, a former chief executive of Hewlett-Packard, has been a prominent Cruz ally since shortly after abandoning her own presidential bid earlier in the year. She was the only woman in the Republican Party's crowded 2016 field.

"Of all the people who didn't make it far in the race, she was one of the best about laying out her plan, talking about who she is and her accomplishments," said Doug De Groote, a fundraiser for Cruz based near Los Angeles.

Fiorina's selection marked another extraordinary development in the 2016 Republican campaign, particularly for a candidate who is far from becoming his party's presumptive nominee. Cruz was soundly defeated by GOP front-runner Donald Trump in all five primaries contests on Tuesday, and he's been mathematically eliminated from winning the nomination before his party's national convention in July.

Some Cruz allies praised Fiorina's selection, but privately questioned if it would change the trajectory of the race. Trump has won 77 per cent of the delegates he needs to claim the nomination, and a win next week in Indiana will keep him on a firm path to do so.

Cruz was to appear Wednesday afternoon with Fiorina in Indiana's capital city, having staked his candidacy on a win in the state's primary contest next Tuesday. Fiorina's California ties could also prove valuable in that state's high-stakes primary on June 7.

"Carly has incredible appeal to so many people, especially in California," De Groote said. "She can really help him here."

Her first major foray into politics was in 2010, when she ran for Senate in California and lost to incumbent Sen. Barbara Boxer by 10 percentage points. She has never held elected office.

Trump criticized a Fiorina pick as "ridiculous" and "dumb" even before it was announced.

"First of all, he shouldn't be naming anybody because he doesn't even have a chance," the New York billionaire said in a Wednesday interview on Fox News.

"Naming Carly's dumb, because Carly didn't do well. She had one good debate -- not against me by the way, because I had an unblemished record of victories during debates -- but she had one victory on the smaller stage and that was it," Trump said.

He added, "She's a nice woman. I think that it's not going to help him at all."

Throughout her presidential bid, Fiorina emphasized her meteoric rise in the business world. A Stanford University graduate, she started her career as a secretary, earned an MBA and worked her way up at AT&T to become a senior executive at the telecom leader.

She was also dogged by questions about her record at Hewlett-Packard, where she was hired as CEO in 1999. She was fired six years later, after leading a major merger with Compaq and laying off 30,000 workers.

Democrats quickly attacked the Cruz-Fiorina alliance.

"The best way to describe that ticket is mean and meaner," said Sen. Barbara Boxer, D-Calif., who beat Fiorina for Senate in 2010. "He wants to throw people out of the country and she threw thousands of jobs out of the country. Perfect match."

In an Associated Press-GfK poll conducted in December 2015, Republican voters were more likely to say they had a favourable than an unfavourable view of Fiorina by a 47 per cent to 20 per cent margin, with 32 per cent unable to give a rating.

Among all Americans, 45 per cent didn't know enough about Fiorina to rate her, while 22 per cent rated her favourably and 32 per centunfavourably.

By contrast, both Cruz and Trump have high negative ratings even within their own party, according to an April AP-GfK poll. Among Republican voters, 52 per cent have a favourable and 41 per cent have an unfavourable opinion of Cruz, while 53 per cent have a favourable and 46 per cent have an unfavourable opinion of Trump.

Among all Americans, 59 per cent had an unfavourable opinion of Cruz and 69 per cent said that of Trump.

Cruz supporter Jim McAdams, who was wearing a homemade "Ted Cruz 16" shirt to the announcement event in Indianapolis, predicted the Fiorina pick would generate badly needed momentum. But the 74-year-old retired mechanical engineer conceded that Cruz is a longshot at this point.

"The only way he's going to get in office is divine intervention," McAdams said. "We pray for his campaign every day."