Donald Trump appears to be pulling away from Sen. Ted Cruz in a key state the senator has called "absolutely pivotal."

A new NBC/Wall Street Journal survey of Republican primary voters in Indiana released Sunday showed Trump with a 15-point lead over Cruz in the Hoosier State. Ohio Gov. John Kasich received 13% support.

The poll is the latest in a slew of surveys that have found Trump expanding his lead in Indiana.

A Fox News poll conducted in mid-April found Trump with 41%, compared to Cruz's 33%. Kasich also garnered 13% in the Fox News poll.

With only a handful of Republican-primary contests looming, it is mathematically impossible for Cruz and Kasich to win the number of delegates needed to secure the nomination on the first round of voting at the Republican National Convention in July. Now, both campaigns are attempting to deny Trump the necessary 1,237 delegates needed to capture the nomination.

Cruz has fought particularly hard to win the Hoosier State, which is seen as demographically more favorable to him than other remaining primary states.

Last week, the Texas senator announced an alliance with his GOP rival Kasich. The Cruz campaign asserted that Kasich would be pulling resources and ads from Indiana in order to allow Cruz to consolidate Republican voters casting ballots against Trump.

But the message from Kasich's camp was less clear, as the governor said he was not encouraging supporters not to vote for him.

"I've never told them not to vote for me," Kasich said during a campaign stop at an Philadelphia diner. "They ought to vote for me. But I'm not over there campaigning and spending resources."

Cruz also chose Indiana as the state to announce his vice presidential running mate, former Hewlett-Packard CEO Carly Fiorina, should Cruz secure the nomination.

At least some polls show Cruz still within striking distance of Trump. The Real Clear Politics average of recent polls puts Cruz behind Trump by only four points.

Still, Cruz already appears to be looking past the state in some ways.

After briefly touting Indiana Gov. Mike Pence's tepid message of support, Cruz on Sunday bestowed the title of the most important upcoming primary contest on another state.

"California is likely to decide this entire battle," Cruz told CNN's Jake Tapper.

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