Republican Senate candidates across the country are battling to emerge as the pro-Trump candidate in contested primaries.

President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s approval rating might be stagnant nationally, but he still enjoys strong support with many of the GOP primary voters who will decide crucial Senate primaries ahead of the 2018 midterm election.

So Republican candidates in states that Trump carried in November are beginning to hammer their opponents as unfaithful supporters of the president to gain an edge in their primaries.

One clear example of this trend is in Indiana, where fellow Indiana Republican congressmen Luke Messer and Todd Rokita are poised to clash in what’s already become a nasty primary. Messer jumped into the race this week, with Rokita all but assured to follow.

Rex Early and Tony Samuel, who served as the top leadership on Trump’s Indiana campaign operation, penned a letter on Thursday hailing Rokita as a loyal Trump supporter throughout the election. While the men wrote the letter in their personal capacity — the dormant Trump campaign hasn’t weighed in — the letter was clearly meant to give Rokita room to claim the pro-Trump mantle.

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“Of the members of Indiana’s congressional delegation, Todd Rokita is the only one that actively and specifically campaigned for candidate Trump — never wavering and never jumping on and off the Trump Train,” they wrote.

Rokita’s campaign manager, Bryan Reed, took the issue further in his own statement that noted The Hill’s reporting that in April 2016, Messer floated the possibility that a white knight candidate could emerge if no candidate won the majority of GOP convention delegates.

“Despite his efforts to fool Hoosier Republicans and cozy up to President Trump now, what is very clear is that Messer was less than enthusiastic, and even openly critical of Donald Trump even as he ran against Crooked Hillary, the media establishment and the beltway elite,” Reed said.

A Rokita ally told The Hill that the campaign would “no doubt” bring up the issue in advertisements if he decides to run.

But Messer’s camp swatted aside the characterization. Greg Pence, Messer’s finance director and the brother of Vice President Pence, praised both Early and Samuel as friends but said that there’s been a “misunderstanding.”

“Rex and Tony don’t speak for the Trump campaign ... it’s not a correct statement and it was made by two individuals who, at this point, are no longer affiliated with the Trump campaign,” Greg Pence told The Hill.

Pence went on to note that Rokita had first endorsed Rubio and added that many members of Messer’s finance committee are longtime allies of the vice president, who served as governor of Indiana for four years.

“Luke voted for Donald Trump in the primary. From my perspective, he’s been standing by him since then,” Pence added.

Trump is still extremely popular among Indiana Republicans — 83 percent of state Republicans approve of his job performance, according to an internal Messer poll published by Howey Politics earlier this month. That’s far higher than Trump’s job approval rating in the state, which Gallup recently found to be 47 percent.

That’s why one unaffiliated Indiana Republican told The Hill that right now, “aligning with Trump is a no-brainer strategy.”

“Who's to say where the trend lines will be come [the day of the Indiana Senate primary], but right now it's where the smart money sits,” he said.

But he noted that Rokita’s release tarred John Hammond, Indiana’s Republican national committeeman, and warned that pressing the issue too hard could backfire.

“The question becomes how far will the game of one-upmanship go and to what end?”

Similar battles are playing out in other states too.

Support for Trump has become the defining issue in Alabama’s upcoming Senate primary, where Sen. Luther Strange (R-Ala.) and his allies are pummeling Rep. Mo Brooks Morris (Mo) Jackson BrooksOvernight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals,' official says MORE (R-Ala.) for critical comments he made about Trump during the 2016 primary.

The attacks, levied by Strange’s campaign and allies like the Senate Leadership Fund, which has earmarked up to $10 million to support Strange in the bid to permanently fill Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE’s former Senate seat, have so far dominated the race. And this week, Brooks doubled down on his comments to side with Sessions, who has been publicly pilloried by Trump.

In Michigan, former Trump state campaign co-chairwoman Lena Epstein has made her support of Trump a key part of her primary bid. Barring any long-shot bid by musician Kid Rock, Epstein has a clear lane as the pro-Trump candidate in the race thanks to her campaign experience.

She’s campaigned as an unabashed supporter of Trump — she signs emails to her supporters as “2016 Trump co-chairman," called for Attorney General Jeff Sessions to resign because of Trump’s disappointment in him and regularly promises to support Trump’s “America First" agenda. And she used that experience to needle her opponent as being the choice of the GOP insiders, comparing the dynamic to how the party insiders had wanted Trump to lose, too.

The dynamic could also play out in West Virginia’s Republican primary, where Rep. Evan Jenkins and Attorney General Patrick Morrisey are locked in their own harsh battle for the party’s Senate nomination. Trump sports a 60 percent job approval rating among all adults there, according to a recent Gallup poll. While the polling doesn’t include a partisan breakdown, the result is indicative of overwhelming support among Republicans.

Jenkins endorsed Trump about a week before the state’s GOP primary, which he won handily. But Morrisey remained neutral until around the GOP convention.

A source close to Jenkins said that they too may focus on the issue in the primary.

GOP operatives agree that there’s ample space to win a party primary as the pro-Trump candidate as long as a candidate isn’t all talk.

“One of the things GOP primary voters liked about Trump was his authenticity, to say what he means and not back down,” said Kevin McLaughlin, a former top adviser to the National Republican Senatorial Committee.

“You have to embrace the style, the overarching theme of ‘We are going to change Washington.’ But at the same time, you have to be authentic to your beliefs.”

But with so much time before the 2018 primaries, let alone the general election, bear-hugging Trump could have its downsides. The president’s approval rating is already underwater in states like Indiana and Michigan, so a full embrace could leave a candidate with little room to moderate if needed.

Bernie Porn, a pollster with EPIC-MRA in Michigan, told The Hill that his late May polling found that Trump has a 67 percent job approval rating among Republicans in the state.

“They may not be aware that the numbers are not as high as they may expect among Republicans, so that might not be the wisest thing in the world,” Porn said about candidates embracing Trump in a purple state like Michigan.

“If they get the nomination, they have to live with that identification with independents.”

But John Yob, an adviser to Epstein’s campaign in Michigan, said that he’s not worried about that dynamic.

“Lena Epstein was Donald Trump's Co-Chairman when he won Michigan for Republicans for the first time since 1988 despite public polling to the contrary,” he told The Hill by email.

“The path for a Republican US Senate candidate to win Michigan in 2018 is with an America First message that emphasizes rebuilding the middle class.”

--This article was updated to reflect that Morrisey remained neutral in the primary.