Donald Trump could take another decisive step towards the Republican nomination for president in the next 24 hours.

His chief rival Ted Cruz has staked everything on winning Tuesday’s primary in the mid-western evangelical heartland of Indiana.

But polls show he might come up short against Mr Trump again – and even big supporters say that might doom his campaign.

Conservative talk show host Pat Miller, a Cruz fan who compered a rally for him in Fort Wayne, told Sky News: “If Ted Cruz doesn’t win Indiana, then he is going to have sit down and do some maths and see if there is any outside shot at all.

“Ted Cruz is very dedicated, he is going to push open every door he can, but he’s also not stupid.”

However, Mr Miller believes victory for Mr Cruz would set him up for a scrap for delegate votes with Mr Trump at the party’s convention in Cleveland in July.

Republican presidential candidate Trump speaks at a campaign event at The Palladium at the Center for Performing Arts in Carmel, Indiana

Donald Trump has increased his lead in recent primaries

No one really knows what that would look like, how it would work or how Mr Trump would react to being robbed of the nomination by the party machine.

The billionaire continues to dominate the race and believes he is now what he terms the “presumptive nominee”.

Mr Cruz’s campaign has generally fared better in states with a high proportion of voters who identify as evangelical. They are often wary of Trump.

Jim DeCamp, a former Army chaplain who is now chairman of Indiana’s Pastors’ Alliance, said: “There are things that we think of when we think of the ingredients of what made this country great and I’m looking for someone who is going to champion what made this country great.”

Ted Cruz embodies those qualities, he added.

During a campaign stop at a coffee shop, Mr Cruz’s wife Heidi and his prospective running mate Carly Fiorina declined to answer questions about his chances.

The candidate himself dismissed recent opinion polls.

He said: “The polls are everywhere.

“I’ll tell you where this race is: this race is tied in the state of Indiana, it is neck and neck and it depends on turnout.”

Supporters hope victory in Indiana would be a springboard to generate donors and support in California, the giant prize which will vote next month.

Mr Cruz appears determined to stay in until Cleveland and fight until the last – defeat in Indiana might make the pressure intolerable.