INDIANAPOLIS (Reuters) - U.S. Republican presidential candidate Ted Cruz, in a last-ditch bid to slow front-runner Donald Trump’s momentum, named former business executive Carly Fiorina on Wednesday as his vice presidential running mate should he win the nomination.

After crushing losses to Trump in five nominating contests in the Northeast on Tuesday, Cruz praised Fiorina, a former presidential rival, as a principled fighter for conservative values who would be a valuable ally on the campaign trail.

“Carly is a vice presidential nominee who I think is superbly skilled, superbly gifted at helping unite this party,” the U.S. senator from Texas told a rally in Indianapolis, the capital of Indiana.

The Midwestern state is the next battleground for selecting the Republican and Democratic presidential candidates for the Nov. 8 presidential election and is shaping up to be Cruz’s best - and perhaps last - chance to block Trump’s march to the nomination.

Cruz acknowledged it was unusual to choose a running mate so early in the race. Traditionally, the winners of the Republican and Democratic nominating races announce their running mates in the period between clinching the nomination and summer’s national conventions.

“I think all would acknowledge this race, if anything, it is unusual,” said Cruz, 45, adding he wanted to give voters a clear choice and offer a rebuttal to media suggestions the Republican race was over.

At a nearby rally in Indianapolis afterward, Trump was scornful of Cruz’s move, calling his rival “desperate.”

In the Democratic race, Hillary Clinton, 68, won four of Tuesday’s five contests, building a virtually insurmountable lead over rival Bernie Sanders, a 74-year-old U.S. senator from Vermont, who vowed to keep fighting until the July convention.

In tapping Fiorina as his No. 2, Cruz was apparently trying to recover from Trump’s landslide primary wins on Tuesday in Pennsylvania, Maryland, Delaware, Connecticut and Rhode Island. The victories made the uphill climb toward the nomination even steeper for Cruz.

“I’ve had tough fights all my life,” Fiorina said after Cruz introduced her. “Tough fights don’t worry me a bit.”

Fiorina, 61, dropped her own White House bid in February after a seventh-place finish in New Hampshire. She endorsed Cruz a month later and has been a sharp critic of Clinton.

Fiorina and Cruz both said they had grown close since her endorsement, and she sang a few verses of a song she said she sings to Cruz’s two daughters on their campaign bus.

The choice of Fiorina, a former Hewlett-Packard Co chief executive who, like Trump, has never held elective office, could help Cruz with women voters, a group that Trump has had difficulty winning over to his outsider campaign.

It also could offer Cruz a boost in the June 7 primary in California, where Fiorina won the Republican primary in 2010 for the U.S. Senate. She was defeated in the general election by incumbent Democrat Barbara Boxer. Fiorina now lives in Virginia.

Trump has amassed 987 of the 1,237 Republican national convention delegates he needs to secure the nomination, to 562 for Cruz and 153 for Ohio Governor John Kasich, 63, the third remaining candidate, according to the Associated Press.

If Cruz can win a large share of Indiana’s 57 delegates, it will increase the chances that Trump will not be able to win the nomination on the convention’s first ballot. After that, many delegates will be free to turn to Cruz, Kasich or a dark-horse establishment candidate on a second or subsequent ballot.

A loss to Trump in Indiana would effectively cripple Cruz’s bid, and increase pressure on the party to rally around Trump as the prospective nominee.

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‘WASTE OF TIME’

Trump called Cruz’s decision to pick a running mate “a pure waste of time” and “a desperate attempt to save a failing campaign by an all-talk, no-action politician.”

“He is the first presidential candidate in the history of this country who’s mathematically eliminated from becoming president who chose a vice presidential candidate. ... It’s a record,” the New York billionaire and former reality TV star told supporters in Indianapolis on Wednesday.

Trump vowed to campaign extensively in the state between now and Tuesday’s nominating contest.

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“I’m not playing games with Indiana,” he said. “You are going to get so tired of me.”

“We’re going to win it,” he said.

Trump was introduced at the rally by famed college basketball coach Bob Knight, who led Indiana University to three national championships.

Cruz said Fiorina had already proven her mettle in standing up to Trump last year after he insulted her looks in an interview. “Look at that face. Would anyone vote for that?” Trump had asked, only to be sternly rebuked by Fiorina in their next presidential debate.

“She’s faced challenges a lot worse than someone bellowing and yelling and insulting her face,” Cruz told the Indianapolis crowd.

A breast cancer survivor who lost a stepdaughter to drug addiction, Fiorina served as Hewlett-Packard chief executive from 1999 to 2005. She was forced to resign amid weak earnings as the company struggled to digest a $19 billion merger with Compaq Computer Corp. About 30,000 layoffs took place during her tenure.

Fiorina’s campaign never took off in the original 17-member Republican presidential field, and she was mostly relegated to the early second-tier debates for low-polling candidates.

Craig Robinson, a former political director of the Iowa Republican Party, said Cruz would have been better off picking an experienced politician who could help him secure delegates in a fight at the convention.

“It’s a real head-scratcher,” he said.

But some conservatives applauded Cruz’s choice. Marjorie Dannenfelser, president of the Susan B. Anthony List, which works to elect political candidates who oppose abortion rights, said Fiorina was the “ideal” choice.

“She will take Hillary Clinton head-on,” Dannenfelser said in a statement.