Sen. John McCain gave an evasive answer this week when asked if he was comfortable with Donald Trump having control of the nuclear codes – another uncomfortable moment for the longtime lawmaker who has tried to walk a thin line between distancing himself from the Republican nominee's more inflammatory statements while not angering his own voters ahead of what may be his toughest re-election fight ever.

Responding to a question in Arizona, where he'll face off with former state Sen. Kelli Ward in a primary at the end of the month, McCain avoided discussing his own feelings on Trump's suitability for taking charge of the nuclear arsenal.

"Anyone that the people of this country choose to be the commander-in-chief and the president of the United States, therefore can lead this country and will lead in a responsible fashion," McCain said.

"That's the way that our democratic system works. That's how our government works," he continued. "The American people select the next president of the United States, knowing full well what role the commander-in-chief is. Therefore, I have the utmost respect for the verdict of the people."

The Arizona Republican has suffered indignity at the hands of Trump since the very earliest days of the former reality television star's campaign, most notably last July when Trump denigrated McCain's military service record by saying he prefers people "who weren't captured" to be considered war heroes.

For his part, McCain – a prisoner of war who underwent torture at the hands of his Vietnamese captors – has spoken out on multiple occasions against Trump, including by rebuking him after the billionaire businessman claimed during a February debate he would employ waterboarding and "a hell of a lot worse" against terrorists. Nevertheless, McCain this spring said he would support Trump in the general election.

But earlier this week, after McCain criticized Trump over his sparring with the Muslim-American parents of a soldier killed in Iraq, the Republican nominee pointedly refused to endorse McCain in his re-election bid.

And Thursday, Trump's running mate, Mike Pence, appeared to follow suit.

"I look forward to supporting Republican candidates in the days and weeks ahead all over the country, and so does Donald Trump," the Indiana governor told NBC News . "But the stakes in this election are so high. To restore our country at home and abroad, we need new leadership, and I'm looking forward to standing shoulder-to-shoulder with Donald Trump to drive that new leadership forward."

Shortly afterward, an aide for Pence walked back the comments, telling Politico Pence supports all sitting members of the GOP in their primaries and his desire for "new leadership" was a reference to himself and Trump winning in November.

McCain's relationship with Trump puts him in a particularly difficult position. If he continues to support the GOP nominee, he risks going down in the general election against Democratic Rep. Ann Kirkpatrick, who is within striking distance of unseating him in recent polls. Arizona looks to be unexpectedly close in the presidential race as well, with the state's high concentration of Latinos threatening to tip the typically GOP state to Democratic nominee Hillary Clinton.

But McCain is also under growing pressure to rescind his endorsement of Trump, including from veterans groups who have denounced Trump's feud with the slain soldier's family.

Alex McCoy – leader of the anti-Trump group Veterans Vs. Hate and a retired Marine sergeant – joined with a handful of other veterans on Thursday to deliver a MoveOn.org petition to McCain's Capitol Hill office, calling on the senator to un-endorse Trump.

The petition has garnered around 100,000 signatures online, and McCoy says McCain has been tarnished by refusing to completely disassociate himself from Trump.

"If there's an opportunity to repudiate Trump, this past week is it," McCoy says. "It's clear that Donald Trump is not going to get better at this. … This is only going to get worse, and the longer he waits the more he will be tarnished, not just in his moral values but in the eyes of voters."