President Donald Trump’s anticipated swing through Arizona in the lead-up to the midterm election is intended to shore up any cracks in the red wall of Republican voters whose turnout in November will be critical to their hold on the U.S. Senate seat.

But it doesn't come without risks for Republicans in every statewide race down ballot.

In his 2016 presidential campaign, Trump used Arizona as a backdrop to make the case for his divisive border-wall proposal and promised crackdowns on illegal immigration.

As Trump approaches the midway mark of his first term, there seems to be a reluctance to return to Arizona.

Twice in recent weeks, Trump’s campaign penciled in tentative stops in Arizona and then canceled them.

While the White House and Arizona Republicans aren't revealing their calculations, the dynamics at work are well-known.

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In Arizona, Republicans worry that Democratic enthusiasm — driven by Trump's unpopularity — could cost them statewide offices, including governor, and possibly control of the state Senate. For them, having Trump here doesn't necessarily help.

Nationally, however, Republicans are bracing for heavy losses that imperil their control of both chambers of Congress. They see Trump's popularity with the GOP base as key to increasing turnout and keeping those losses to a minimum.

High stakes for Martha McSally

The president's on-again, off-again plans come at a critical moment for Republicans, in particular U.S. Rep. Martha McSally, who has leaned on her relationship with the president in her quest for the state's open Senate seat.

U.S. Sen. Jeff Flake, one of the president's most vocal Republican critics, is not running for re-election, at least partly because his public antagonism toward Trump made him a pariah among GOP primary voters.

McSally, a two-term congresswoman from Tucson, is running against Democratic U.S. Rep. Kyrsten Sinema, a three-term congresswoman from Phoenix, to replace Flake.

Both women are considered top recruits for the seat by their party bosses and the seat is deemed a toss-up by political analysts who see Republicans' hold on the seat as vulnerable.

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Trump endorsed McSally right after the Aug. 28 primary, calling her an "extraordinary woman" who has his full support in the race.

If he appears in Arizona before November, Republican strategists and those familiar with the planning have said McSally would share the stage with him.

The founder of her campaign's Missouri-based consulting firm, Axiom Strategies, argues Republicans should embrace Trump.

"The Republican Party will either rally around its Republican president — and defend our shared accomplishments — or it will enter the fall with a depressed base and a turnout disadvantage that will lead to major losses in Congress, governorships and state legislatures," Jeff Roe wrote in a March opinion piece for The New York Times.

"Manageable losses or catastrophic defeat: What will it be, my fellow Republicans? My answer: Fix bayonets and charge the hill."

Roe is the campaign manager for U.S. Sen. Ted Cruz, R-Texas, a one-time Trump opponent in the 2016 primaries who this year is facing a serious challenge from Democratic U.S. Rep. Beto O'Rourke.

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McSally was scheduled to fly to Las Vegas to appear alongside Trump on Friday, at a bill-signing event, her campaign said Thursday.

During a swing through Nevada to help Republican U.S. Sen. Dean Heller, the party's most vulnerable Senate member, Trump was scheduled to sign an appropriations bill for military construction, Veterans Affairs, and other spending items.

By Thursday evening, McSally had canceled those plans because of scheduling issues.

Trump visit could hurt some Republicans

Those familiar with Trump's tentative Arizona rally plans have blamed the cancellations on vague scheduling issues. They have refused to discuss private conversations with the White House and Trump campaign about the political dynamics at play.

A Trump visit to the state would remind Republican voters what’s at stake for the party, while firing up likely GOP voters who showed up for him in 2016 but traditionally don't vote during midterm elections.

But the free-wheeling and sometimes unpredictable rallies that Trump is known for could benefit Democrats by introducing volatility to other statewide campaigns that are scripted and choreographed and where candidates rarely stray from well-worn talking points.

READ MORE: Democrats' turnout helps shatter primary record

A Trump rally in Arizona could further energize Democratic voters’ opposition to the president while driving away suburban women and independent voters, two key demographics that Republicans fret about in a Trump era.

Kevin Madden, who was a spokesman for Mitt Romney's 2008 and 2012 presidential campaigns, said the midterms will come down to which party holds the mantle on voter enthusiasm.

"Republicans are less enthusiastic about voting in this election," he said. "The one thing that's going to juice them and maybe close that gap is maybe their support for the president. Republican candidates right now are being forced to reckon with that ... What's the one thing they believe is actually getting them to the polls? It's a message from the president about the stakes of the election."

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A June survey by Pew Research Center shows high voter engagement in this year’s elections among both Democrats and Republicans compared with prior midterms. A record share of registered voters — 68 percent — said the issue of which party controls Congress with be factored into their vote.

Compared with recent midterms, the Pew survey said, more voters indicated their view of the president will influence their vote for Congress.

Arizona's Senate race is tight

A Trump visit could nationalize the race and distract from local issues and the carefully crafted narratives that Arizona Republican candidates want to talk about.

"The president in some ways can be counterproductive in reminding Democrats and the president's more vocal critics of what's at stake in this election," Madden said.

Polls show the U.S. Senate race is tight, with McSally chipping into Sinema's lead.

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McSally faced two challengers from the right during the primary and spent considerable money and time securing a win.

Sinema faced a challenge from the left but her rival had no money, giving the congresswoman free rein to roll out slick TV ads that introduced her to voters as an "independent" voice who would work with anyone to help Arizona.

The contest has tightened in the early weeks of the general election as McSally and her allies have blasted Sinema's 15-year-old war-protesting activities and comments on a bill involving stricter penalties for people soliciting prostitutes.

James Owens, a spokesman for Sinema, did not answer whether a visit to Arizona by Trump would help or hurt McSally. Instead, he pivoted to McSally's voting record as a reliable vote for Trump and Republicans' legislative agenda, namely health care and Social Security.

By contrast, he said Sinema "isn't afraid to buck her party."

The Senate race is so close, a visit by Trump could tilt the race in either direction, experts say.

With his presidency's agenda at stake, Trump has booked a heavy campaign schedule planned around key races, and those featuring candidates he personally likes.

From Montana to Missouri, Trump is headlining rallies and fundraisers to play up the strong economy, tax cuts and his border wall, while lashing out at the Republican antagonists who aren't on board with his agenda as well as Democrats and the media.

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Jennifer Duffy, who analyzes U.S. Senate races for the influential and nonpartisan "Cook Political Report," said Trump's visits are not just about 2018.

"With Trump, it's about candidates he really likes, and places he feels very invested in — with an eye towards 2020," when he is up for re-election, Duffy said.

"He intends to travel a lot in October, and frankly, the more this (Brett) Kavanaugh Supreme Court nomination is is crazy-land, I think the more he travels. He will start going after Democrats for putting the brakes on this. And it could totally backfire on Democrats," she said.