(CNN) On the Saturday before Utah's primary, Salt Lake County Mayor Ben McAdams was hustling — knocking on doors and on the hunt for persuadable Republicans to support his congressional bid.

Here in Utah's Fourth District, a boot-shaped district covering the suburbs south of Salt Lake City, the primary election Tuesday was a foregone conclusion. The highly competitive race between this popular Democratic mayor and his opponent, Republican Rep. Mia Love, was set this spring when they each secured their party's nomination. Still, recognizing the steep climb he faces to unseat Love, McAdams dashed from house to house, showing the earnest energy and persistence of a former Mormon missionary.

Laced up in a pair of blue Asics that serve as his "parade shoes," McAdams scanned the driveways to game out which voters were home, sometimes hitting doors that weren't on his list. While walking, he scribbled notes on his campaign literature for voters who were out: "Sorry I missed you," he wrote next to his name, tucking it under their doormat.

While much of the attention in Utah has been focused on the reemergence of Mitt Romney, who is running for US Senate, the Love-McAdams race is much more consequential to both parties.

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As Republicans try to maintain control of the House, there are few GOP districts in the country where President Donald Trump could cast as long a shadow as he will here. In 2016, Trump drew just 39% of the vote in Utah's 4th District, to Clinton's 32%, with independent candidate Evan McMullin sweeping up much of the remainder.

The race between these two charismatic players will be one of the most fascinating midterm contests this fall, not only because Love has been one of the most high-profile Republicans to stand up to Trump on immigration, but because national Democrats see McAdams as one of their best chances to gain a foothold on red turf.

Salt Lake County covers 85% of the 4th District, giving McAdams a distinct advantage when it comes to local issues. When an undecided Republican City Councilman passes by, the Mayor notes that he is staying on top of much-needed street repairs on a nearby corridor. When a mother leaned out the car window to offer encouragement—"We love you. We'll do anything we can to help," 38-year-old Candice Jorgensen told him — McAdams leaned in to give her son a tip for scout camp: "Try the mulberry juice."

Though McAdams will be well-funded with strong backing from Democrats nationally, he acknowledges the difficulty of his quest in a state where Republicans outnumber Democrats by more than four to one. Strategists on both sides estimate that McAdams must win a fifth to a quarter of Republican votes to prevail.

Love's challenge will be to turn out Republicans who are not enamored with the President, particularly in more conservative Utah County. Trump's opportunity to nominate a conservative justice to the Supreme Court could help energize the base in Love's district, and she is also likely to get a lift from Romney, the former Republican presidential nominee who is at the top of the ballot as he runs for Utah's open U.S. Senate seat.

Trump's long shadow in Utah

But Trump's crude broadsides and harsh immigration policies have been an affront to Utah's huge population of Mormons, who embrace a doctrine of acceptance and inclusivity toward immigrants.

In such a staunchly Republican state, one would expect a Republican president to have "a substantially higher" approval rating than Trump does, said Brigham Young University Political Science Professor Chris Karpowitz.

Karpowitz notes that while Trump is popular among GOP voters nationally, his approval rating is unusually low among Republicans in Utah.

"He is, in so many ways, the polar opposite of what conservative Mormons are taught to stand up for," Karpowitz said.

The separation of families at the border was particularly abhorrent to Utah Mormons, because the importance of keeping families together is at the core of the doctrine of the Mormon Church. The neighborhood where McAdams knocked on doors this past weekend was dotted with signs supporting immigrants; some placards in the windows opposed the separation of families.

Many Mormon missionaries, Karpowitz added, have served "in places like Mexico, Guatemala, El Salvador, Honduras and Nicaragua. They have a sense of what life is like on the ground in those countries — and why refugees might be trying to get in to the United States."

Love was among the first Republicans to strongly condemn family separations at the border, calling the policy "absolutely terrible." The Mormon Church also rebuked the Trump administration's "zero-tolerance" policy and "aggressive and insensitive treatment of these families."

The Utah congresswoman argues that she has been an independent voice for Utah, particularly on immigration issues. As the daughter of immigrants from Haiti, she gained national attention when Trump demeaned Haiti and several African nations as "shithole countries."

When asked how she has tried to influence Trump on immigration, Love said bluntly in a telephone interview that she doesn't expect leadership from the President on the issue.

"We cannot depend on the White House to make all these decisions for us," she said. "At one point he said that there was nothing he was going to do about what was happening at the border. The next moment he signs an executive order to stop what's happening at the border. One minute he comes in and says, 'You have to vote on immigration,' the next minute he tweets 'I don't know why you're wasting your time on immigration.'"

"It's completely unpredictable, which is why I keep pushing Paul Ryan and Kevin McCarthy to take the reins and do what they are supposed to do, which is to enact law," Love said.

While Love is creating that distance between herself and the President, McAdams argues that her votes have aligned with the President's position 96% of the time, according to an analysis by FiveThirtyEight.

"The movement to the center that we've seen from her in the last couple of months is nothing more than just show for an election," McAdams said, as he walked through the streets of Millcreek on Saturday. "I don't know how much we should be applauding her recent move to the center in light of her past record and her past statements. How much of this is a real change of heart, and how much is for show?"

Love, who spent last weekend in Washington trying to revive immigration legislation that later failed, chafed at that characterization.

"We have been out front, on our own, many times on immigration, even before he got in the race," Love said. "It's actually incredibly offensive that he would think that I would need to see his face to remind myself that I am a daughter of immigrants. It's absolutely arrogant."

She added that it took "a lot of courage" to buck her party's leadership and sign the discharge petition that has forced the immigration debate on the House floor.

"We were the only ones in the delegation that did it. No one else did," she said. "And I guarantee you, we would not have had a vote last week, and we would not be having a vote this week, if it were not for that discharge petition."

After the immigration compromise bill failed this week, the McAdams campaign criticized Love for not doing more to force immigration legislation through Congress.

"People's lives are in limbo, families are being torn apart, our economy is suffering, and Rep. Love has completely failed to deliver," McAdams' campaign manager Andrew Roberts said.

A Democrat who works with Republicans

Ben McAdams, Democratic candidate for Utah's 4th Congressional District.

Over his two terms as mayor, McAdams has endeared himself to independents and Republicans in Utah with his pragmatic, wonky and slightly quirky style. His campaign bus is bright orange, which he said was a nod to his first job as a 12-year-old picking pumpkins—and also a color that would make people stop and pay attention.

As a state legislator, McAdams put great effort into building relationships with Republican lawmakers. That is now a central theme of his campaign literature: that he is a centrist who "works across party lines, gets things done." His embrace of bipartisanship is generally the first trait he mentions when he tries to woo voters at their doorstep.

Some of his collaborations were cleverly designed stunts. In 2014, for example, he challenged Republican Lt. Gov. Spencer Cox, to face him and other drivers in the Dog Days of Summer Demolition Derby at the Salt Lake County Fair. Local news stations covered the match as the cars rammed into one another. (One of the cars slammed into Cox, flipping his vehicle upside down ).

McAdams worked closely with Cox and Republican Speaker of the House Greg Hughes on the growing homeless problems in Salt Lake City. Later Hughes and McAdams taped a "carpool karaoke" version of "Hamilton" for YouTube.

The Salt Lake County mayor also showed an unusually hands on approach to policy when he secretly went undercover as a homeless person for three days as a fact-finding mission while searching for a location for a new shelter.

The race heats up

As the Love-McAdams race heats up, Republicans are attempting to frame the mayor as a hardcore partisan who can't be trusted. The National Republican Congressional Committee launched a website last year that drew attention to a White House internship that he did while Bill Clinton was President.

Attempting to put some distance between himself and the liberals within his party, McAdams says he won't support Nancy Pelosi to lead his caucus, because he wants to see "new leadership."

He has a similarly cautious answer when asked whether he would favor Trump's impeachment if Democrats take control of the House.

"Let's see what the facts are first," McAdams said. "I think the best way to remove a President is through an election. So let's see what happens to the investigation. But you have to start with the baseline that he was elected through our processes, and he's going to be the President unless there were high crimes and misdemeanors—and we don't have any information on that yet."

Love has also highlighted a vote that McAdams took on abortion as a state lawmaker, suggesting in a stinging speech at this year's GOP convention that he would use the 4th District congressional seat to support abortion.

McAdams responded over Twitter: "As an active Mormon, I find this attack offensive & not the way we do things in Utah. Typical mudslinging from a typical Washington insider."

As an active Mormon, I find this attack offensive & not the way we do things in Utah. Typical mudslinging from a typical Washington insider. If she spent time listening to our issues, she could talk about that. We deserve better than partisan rhetoric and personal attacks. #utpol https://t.co/hP0eBQghm4 — Ben McAdams (@BenMcAdams) April 14, 2018

The Salt Lake County Mayor, in turn, contends that the 4th Congressional District seat has been an "empty chair."

He accused Love, for example, of failing to help the state get waivers from the Department of Health and Human Services for Medicaid expansion.

Love retorted that he wasn't aware of her work on the issue, because he wasn't engaged, adding that McAdams never called her—or his own Congressman—asking them to assist on that issue.

The two-term congresswoman frequently calls attention to the fact that McAdams does not live in the 4th District (though living in the district is not a requirement for House candidates). She folded the residency issue into her criticism of his initial support for a high-density housing development known as the Olympia Hills project, which sparked outcry among local residents in southwest Salt Lake County.

Accusing McAdams of taking campaign donations from developers linked to the project, Love argued it was an example of his "pay to play" tactics.

"It shows that when you don't live among these communities, you always make horrible decisions," Love said in a telephone interview Monday. "The West side (of the county) has always been screwed over by Ben." (McAdams says he is now working to reduce the scale of the project).

As Republicans attempt to paint him as a Democratic partisan, McAdams has been telling voters that he's willing to work with the GOP in Washington, including Trump.

"I think people are tired of partisanship and gridlock, and I'm going to be the same person in Washington as I have been as mayor and in the state senate," McAdams said. "Trump is going to be right. And where he's right, I'm going to work with him, and where he's wrong, I won't."