You’d think John McCain would be a shoo-in for reelection. The five-term Senator was a war hero, the Republican presidential nominee in 2008, and has won each of his congressional elections over his 34-year career by at least 10 percentage points.

But this re-election cycle may be his toughest ever, he said.

Yes, Sen. McCain is predicted to beat his primary opponent in Arizona on Tuesday. But the margin by which he wins – whether he handily defeats Kelli Ward, or just sneaks by her – could indicate how much of a fight he will have against Democratic Rep. Ann Kirkpatrick.

Even as he braces for the race, McCain is struggling to balance support for Donald Trump, his party’s nominee, while staying true to his own stance on issues such as immigration reform and gun control. McCain is one of many incumbents nationwide who have faced a rough road to re-election in a tumultuous season that has seen the rise of Mr. Trump and Democratic Sen. Bernie Sanders of Vermont.

McCain led Dr. Ward, an osteopathic physician, by 26 points two weeks ago in a CNN/ORC poll, with a margin of error of plus or minus 5 points, according to CNN. But that doesn't mean this primary has been easy.

Ward has criticized the octogenarian's age and has tried to lure conservatives by casting doubt on McCain’s support for Trump, arguing he could abandon the nominee in the general election, according to Politico. McCain has denied this claim even though Trump belittled him during the primary, saying that McCain was only considered "a war hero because was captured. I like people who weren't captured."

Some conservatives also consider McCain too liberal on immigration, campaign finance reform, and gun control, according to The Republic newspaper in Arizona. Although McCain has faced this criticism before, navigating this balancing act this time around could prove tricky.

Kirkpatrick is already making the exact opposite argument from Ward. McCain, Kirkpatrick says, has lost his independent streak by refusing to disavow Trump, as Politico’s Burgess Everett writes.

In Kirkpatrick’s corner is the changing makeup of Arizona. Once a GOP stronghold, the state’s electorate has seen an increase in Latino voters. Even though McCain has been a champion of immigration reform, Trump hasn't. The presidential nominee has said he will build a wall along the Mexican border and forcibly remove 11 million undocumented immigrants.

Nationwide, four incumbents have already lost this year, according to CNN. Republican Reps. Renee Ellmers of North Carolina, Randy Forbes of Virginia, and Tim Huelskamp of Kansas, as well as Democratic Rep. Chaka Fattah, who was convicted of federal corruption charges in June, all lost in the primaries.

In Florida, Rep. Debbie Wasserman Schultz is facing Tim Canova, a law professor backed by Mr. Sanders. Ms. Schultz faces criticism that as chair of the Democratic National Committee, she gave a boost to Hillary Clinton over Sanders.

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Because McCain is considered a centrist Republican, he has never had all of the GOP behind him. But that weakness often turns into a strength as November approaches.

“John McCain doesn’t win primaries with big splashes. He’s never going to win 70-30,” longtime adviser Charlie Black told Politico. “McCain’s a bigger figure in Arizona than Trump or [Hillary] Clinton. I’m not too worried about it right now.”