His firebrand campaign close to a cliff edge, Senator Ted Cruz has rushed to protect his home flank, imploring a ballroom filled with several hundred supporters in South Dallas to stay at his side as his own state of Texas and 10 others vote on Super Tuesday, a vital milestone in the nomination race.

In a searing speech, Mr Cruz repeatedly assailed Donald Trump, who appears set to win if not Texas, where Mr Cruz holds a lead according to polls, then perhaps most of the other states voting on Tuesday, inflicting still further damage on both his own campaign and that of Senator Marco Rubio of Florida.

“Super Tuesday is the most important day in this entire primary season. We are going to have a very good Super Tuesday,” he said, before virtually begging the room to help build turnout among his supporters in Texas and beyond. Should he lose in Texas, Mr Cruz would face pressure to leave the race.

“If you don’t think Donald Trump is the right candidate to go head to head with Hillary Clinton then this is the only campaign that has gone against Hillary Clinton and the only campaign that can beat Hillary Clinton,” he declared.

But it was at Mr Trump that he aimed his cannons, accusing him of being naive on Middle East policy, giving donations to Democrats who backed immigration reform in Washington in 2013 and hiring “hundreds of foreign workers” instead of Americans at his resorts.

“You don’t get to abuse and take advantage of American workers and suddenly style yourself a champion of American workers,” he said.

Nothing is predictable in this election. If Mr Trump falls short of expectations today, it is likely to have less to do with the strength of his rivals and more with his stumbles, including his failure in a CNN interview on Sunday to condemn the Ku Klux Klan and spurn the endorsement from the former KKK Grand Dragon David Duke. Yesterday, he claimed he had not understood the question, because of a faulty earpiece.

In pictures: US Elections 2016 Show all 15 1 /15 In pictures: US Elections 2016 In pictures: US Elections 2016 Democratic U.S. presidential candidate Bernie Sanders speaks to supporters after rival candidate Hillary Clinton was projected as the winner in the Nevada Democratic caucuses Reuters In pictures: US Elections 2016 Democratic presidential candidate Hillary Clinton takes photos with workers at her campaign office in Des Moines, Iowa AP In pictures: US Elections 2016 Republican presidential candidate Mike Huckabee, second from left, prays before lunch with supporters at Drake Diner in Des Moines, Iowa Reuters In pictures: US Elections 2016 Democratic presidential candidate and former Maryland Governor. Martin O'Malley, speaks during a campaign stop in Waterloo, Iowa AP In pictures: US Elections 2016 Democratic presidential candidate Bernie Sanders speaks, as his wife Jane O’Meara Sanders looks on, at a campaign event at Iowa State University Getty In pictures: US Elections 2016 Republican presidential candidate Marco Rubio speaks at St. Ambrose University in Davenport, Iowa Reuters In pictures: US Elections 2016 Republican presidential candidate Ben Carson speaks at a campaign event at Fireside Pub and Steak House in Manchester, Iowa. Getty In pictures: US Elections 2016 Republican presidential candidate Rick Santorum visiting supporters at a house party in West Des Moines, Iowa Reuters In pictures: US Elections 2016 Republican presidential candidate Donald Trump at a campaign event in Cedar Rapids, Iowa AP In pictures: US Elections 2016 Republican candidate Ted Cruz campaigns at Greene County Community Centre in Jefferson, Iowa AP In pictures: US Elections 2016 Senator Rand Paul speaks during a Caucus rally at his Des Moines headquarters in Iowa Getty In pictures: US Elections 2016 Republican candidate Jeb Bush speaks at a campaign event in Des Moines, Iowa AFP In pictures: US Elections 2016 Former Alaska Governor Sarah Palin introducing the arrival of Donald Trump during a campaign rally in Cedar Rapids, Iowa Reuters In pictures: US Elections 2016 A portrait of Democratic candidate Bernie Sanders at his campaign headquarters in Des Moines, Iowa Getty In pictures: US Elections 2016 Campaign badges on sale ahead of a Trump rally at the Ramada Waterloo Hotel and Convention Centre in Waterloo, Iowa Getty

Mr Cruz has built his campaign on a southern strategy from the start, which either crumbles or bears fruit today when, aside from Texas, a welter of southern states filled with the evangelical Christians he would normally count on, including Alabama, Arkansas, Georgia and Oklahoma, also vote. Yet that base of support has been plundered by Mr Trump, who has outperformed Mr Cruz among evangelicals and conservatives in those states that have already voted. In Nevada last week, Mr Trump won 40 per cent of the evangelical vote, further sabotaging the main strategic premise of Mr Cruz: that born-again Christians would rise up, across the South especially, and carry him to the nomination.

For Republicans still not convinced by Mr Trump, or just despairing of him, the options are running out. Among those who filled the Gilles Ballroom in Dallas for the Cruz event, Cynthia Stewart, a retired college administrator, said she thought it unlikely the Texas Senator or anyone else would be able to stop the Trump train. But then she could barely imagine that might mean voting for Mr Trump in November.

“For me it will always be anybody but Hillary,” she said. “But my goodness, I’m afraid it is going to come down to Donald Trump. He will have so many pitfalls, but I may have to vote for him.”

“It’s very depressing,” added Gary Rains, 56, a reservoir engineer, a self-described constitutional conservative and Cruz backer. Mr Trump was “brainwashing the uneducated”, he offered. “I don’t know if he is going to be able to keep it up but if Ted Cruz isn’t nominated, I think Trump will be beaten by Hillary Clinton. I can’t see how we can have four years of Clinton – our country will be destroyed”.