They are rivals on the campaign trail, but Republican presidential challengers Donald Trump and Senator Ted Cruz joined forces Wednesday at a rally denouncing the Iran nuclear deal being debated by the US Congress.

Thousands of people converged on the West lawn of the Capitol to hear the two Republican powerhouses and to protest President Barack Obama's landmark accord with Iran, saying the international agreement could mark a death knell for Israel and threaten the lives of millions of Americans.

Trump, the billionaire real estate tycoon and Republican frontrunner, blasted the "incompetently negotiated" deal and criticized the Obama administration for failing to include the return of four Americans held in Iran as part of the negotiation.

"If I win the presidency, I guarantee you that those four prisoners are back in our country before I ever take office," Trump boomed, to raucous applause.

"We are a country that owes $19 trillion," Trump added. We lose everywhere," he added, departing from the Iran rhetoric to repeat his claims that as a world-class negotiator he would get America back on the winning track.

"We will have so much winning if I get elected that you may get bored with winning," he said.

Cruz also received a hero's welcome from the crowd, which was aligned with the conservative grass roots Tea Party movement that helped propel Cruz to victory in his 2012 Senate race.

"If you vote to send billions of dollars to jihadists who have pledged to murder Americans, then you bear direct responsibility for the murders carried out with the dollars you have given them," Cruz said of Democrats supporting the agreement, referring to frozen funds that would return to Iran when sanctions are lifted.

"You can not wash your hands of that blood."

The July deal struck with Iran by the United States and other world powers provides Tehran relief from crippling economic sanctions in exchange for limits on its nuclear program.

Republicans complain the deal does not do away with the program altogether, fails to provide for spot inspections of nuclear sites or force Iran to end support for militant groups like Hamas.

- Trump-Cruz 'bromance' -

Cruz is in the middle of the pack in the race for the Republican nomination. He is one of the few GOP candidates who has refrained from attacking the brash Trump, who routinely antagonizes rival Jeb Bush and has angered Hispanics with his immigration rhetoric.

When Cruz finished his address, he embraced Trump on stage, highlighting a Republican "bromance" that even Trump himself acknowledged.

"It is a little bit of a romance," Trump told CNN after his address.

When Trump criticized US immigration policy, "Ted Cruz was out there, and he really backed me very strongly", the tycoon said, "and I always respected that."

Despite the heated opposition outside the US Capitol, and the ongoing debate inside, the Iran deal has sufficient support to go ahead.

Forty-two Senate Democrats are in favor, more than enough to block a resolution of disapproval against the deal in the 100-member Senate. All Republicans stand opposed.

Rally protesters held up signs attacking Democrats for supporting the controversial accord, which is aimed at rolling back Iran's nuclear program in exchange for an easing of punishing economic sanctions.

"What part of death to America, death to Israel don't you understand?" one sign read.

"Anybody who votes for this deal hates America and wants her to go down in flames," Jim Griffin, a protester dressed in a Captain America outfit and waving a large American flag, told AFP.