President Trump's strategy of becoming aggressively involved in the midterm elections is prompting concern among some Republicans who worry he's complicating the political calculus for GOP candidates trying to outrun his popularity.

Those Republicans worry their statewide candidates may rise or fall based on Mr. Trump's standing, muddling their path to maintain control of Congress.

But Mr. Trump has no plans to step out of the spotlight. He will hold a rally Saturday in Ohio and plans to host two fundraisers at the Trump National Golf Course in Bedminster, New Jersey, next week for House and Senate candidates, according to a campaign official with knowledge of the president's events. The official spoke on the condition of anonymity to discuss details about the fundraisers that haven't yet been publicly released.

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The president is casting himself as the star of the midterms, eagerly inserting himself into hotly contested primaries, headlining rallies in pivotal swing states and increasing his fundraising efforts for Republicans. Last week, Mr. Trump agreed to donate a portion of his reelection fund to 100 GOP candidates running in competitive House and Senate races.

He's expected to be even more aggressive in the fall. White House officials say he's reserving time on his schedule for midterm travel and fundraising likely to surpass that of former presidents George W. Bush and Barack Obama.

"This is now about Donald Trump," said Al Cardenas, a former Florida Republican chairman. "It's a high-risk, high-stakes proposition."

The question facing Republicans is whether turning out those Trump loyalists is enough to win in toss-up congressional districts or if their path to victory depends more on capturing a share of independents and suburban women turned off by Trump's tumultuous first term. It's a dilemma they will confront in 2018 and beyond.

"If we lose the governor's race for the first time in 20 years, all of a sudden President Trump's chances of winning in 2020 diminish with a Democratic governor," said Cardenas. "You can't win a presidential election if you're a Republican without winning Florida."

Mr. Trump's aides argue no one energizes Republicans like the president, pointing to the throngs of thousands who wait in long lines to attend his rallies — he's held 17 since taking office. The aides say the White House is taking a two-pronged approach, sending Trump to mobilize the base while other officials, such as his daughter Ivanka, can generate local headlines and help with voters who may not like the president's aggressive style. The goal is to ensure that the occasional voters who turned out for Mr. Trump in 2016 cast ballots in the midterms.

But there are some signs that Mr. Trump's unpopularity with the general electorate may hamper more than help individual Republican candidates.

While Republicans have won a series of special elections since Mr. Trump took office, they've captured smaller margins than in previous years. Democrats also had two high-profile upsets, nabbing victories in an Alabama Senate race and a Pennsylvania House race.

The GOP is worried about a special congressional election Tuesday in a central Ohio district that Trump won by 11 percentage points in 2016. A Monmouth University poll released this past week showed the race tightening, leaving Republican Troy Balderson with just a 1-point edge. The survey found 46 percent of likely voters approved of Trump, while 49 percent disapproved.

Hoping to shore up GOP support, Mr. Trump plans to host a rally in the district Saturday. His visit follows a Monday stop by Vice President Mike Pence.

The president's team keeps a close eye on data assessing whether Americans believe the country is headed in the right direction under Trump. And they point to Mr. Trump's strength among Republican voters and an upbeat attitude about the nation's economic climate as evidence Republicans will avoid the rough midterm elections that have afflicted previous administrations.

But some Republicans warn Mr. Trump's outsized media presence drowns out the messages of congressional candidates, who believe the path to victory lies with a focus on local issues, the Republican tax cuts and the prospect of Nancy Pelosi becoming House speaker again. In Ohio, Balderson and his GOP allies have tried to tie Democrat Danny O'Connor to Pelosi. O'Connor has repeatedly said he would like to see a new generation of leadership in the House.

"Part of the reason why the Nancy Pelosi attacks are so important is that they're a way to motivate the Republicans who might not love Trump," said Ohio GOP strategist Terry Casey.

Still, Republicans are often forced to fend off questions about Trump-sparked controversies. In recent days, Mr. Trump publicly mused about a government shutdown sometime in the fall — a possibility that Republican congressional leaders fear would significantly hamper their electoral prospects.

In Pennsylvania on Thursday, the president said he was a "little bit torn" about whether it would be better to shut down the government before or after the midterm elections to secure funding for his border wall. "Whether it's before or after, we are getting it or we are closing down government," he told thousands of supporters at a rally in Wilkes-Barre.

That kind of uncertainty only serves to further embolden Mr. Trump's opposition, say Democrats.

"Clearly he lights the fire when it comes to energized Democrats," said Ohio Democratic Party chairman David Pepper. "He's going to come and give a rambling, over-the-top speech that has nothing to do with this district or Troy Balderson. It may hurt more than it helps."

The president's decision to intervene in recent GOP primaries is also raising concerns among some state party officials and politicians, who fear he's siding with candidates who could prove weaker in general elections. Mr. Trump has relished doling out endorsements, sometimes blasting out several a day — even for those who don't need his backing right now.

On Thursday, he tweeted support for Rep. Steve Stivers, an Ohio congressman who chairs the campaign committee for the House GOP, urging people to back him in a primary contest next week. Stivers' primary was held in May and he ran unopposed. The tweet was quickly deleted.

The president has compiled a winning streak in recent primaries in which he has made an endorsement, helping favored candidates in South Carolina, Alabama and Georgia.

His rally this week in Tampa, Florida, represented his most ambitious attempt to nationalize two races crucial to Republicans' midterm hopes.

Trump stood onstage with Rep. Ron DeSantis, a 39-year-old three-term congressman, imploring his supporters to back his campaign for governor. DeSantis was little-known to Republican voters until Trump first tweeted support for him in December. Since then, he's made his ties to Trump a centerpiece of his primary race, focused on Fox News appearances and ads. In recent weeks, he's opened up a double-digit lead over state Agriculture Commissioner Adam Putnam, a longtime fixture in Florida politics.

DeSantis said he was grateful for Mr. Trump's support but added, "I appreciate more the leadership you're showing for our great country."

The president also repeatedly praised Gov. Rick Scott, a Trump ally running for Senate, and attacked his opponent, Democratic Sen. Bill Nelson. Trump, who spends winter weekends at his estate in Palm Beach, claimed the only time he sees the senator is "five months before every election."

"After a while, you forget who's the senator," Trump said.