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By Adam Howard

On Thursday, the two last Republican Party nominees for president -- Arizona Sen. John McCain and Mitt Romney -- will unite for a final get-out the vote effort. The once bitter primary rivals will seek to make the case for McCain to return to the U.S. Senate for a sixth term in a state that has become a presidential battleground in this election cycle.

But don't expect them to make the same argument for Donald Trump's candidacy even as the hours until Election Day tick down.

McCain is a heavy favorite to win but the fate of the state's 11 electoral votes is anything but certain, leaving the duo with the sensitive task of energizing Republicans around McCain's candidacy without publicly supporting the top of the GOP ticket.

McCain has already aired TV ads suggesting that should Democratic nominee Hillary Clinton win the presidency on November 8th, its crucial that a GOP-led Senate stay in power to provide a check against her.

Although Trump openly questioned his war record and expressed skepticism over the heroism of prisoners of war, McCain eventually endorsed him, albeit tepidly, and received Trump's support in exchange.

Related: Ohio Gov. John Kasich, Disliking Trump and Clinton, Votes McCain for President

But after the infamous 2005 "Access Hollywood" taped leaked last month, in which Trump was caught bragging about sexual assaulting women on a open microphone, McCain rescinded his endorsement. He had also previously condemned Trump's attacks on federal judge Gonzalo Curiel and Khizr and Ghazala Khan, a Gold Star family.

Romney, on the other hand, has been one of the most prominent Republican faces of the "Never Trump" movement. Although Romney aggressively sought and touted Trump's endorsement four years ago, he delivered a blistering speech in March, outlining why he believed the real estate mogul was an unacceptable candidate for president, while also declining to throw his weight behind any of the other candidates in the race. He even told NBC's "Today Show" that same month that he'd do everything he could within "political bounds" to stop Trump.

Meanwhile, Trump has been particularly vocal about his disdain for Romney and the campaign he ran in 2012, and he has complained in the past that both he and McCain "let us down" by losing races to President Barack Obama that he believes were easily winnable.

Ironically, several swing state and national polls suggest that Trump has under-performed significantly compared to Romney's 2012 totals with a number of demographic groups.

However, Romney and McCain aren't the only other former GOP presidential standard bearers to steer of Trump, with the exception of 1996 nominee Bob Dole, every other living nominee for president (i.e. former Presidents George H. W. Bush and George W. Bush) have declined to support the party's choice for 2016.