Republicans are concerned Donald Trump will limit candidates’ popularity in the upcoming midterm elections as he parachutes into Ohio, attempting to drag the party across the line in a once-safe seat.

The GOP holds a razor-thin lead in Ohio’s 12th Congressional District as voters head to the polls for a special election on Tuesday in a constituency represented by a Republican for all bar two years since 1939.

Democratic candidate Danny O’Connor has narrowed the gap to Republican rival Troy Balderson to just one per cent according to recent polls of the district, which Mr Trump carried by 11 points in 2016.

The president is expected to hold a rally in Ohio on Saturday in a bid to shore up GOP support just days after his deputy, Mike Pence, also paid a visit to the state to campaign for Mr Balderson.

Democrat Danny O'Connor is in with a shout of winning Ohio's Republican-friendly 12th congressional district (Reuters)

However, Mr Trump’s appearance will do little to ease the worries of those in the party who fear they will suffer another embarrassing special election defeat.

Republicans are growing concerned with the high-profile role the president seemingly intends to play in November’s midterm elections, which could see them lose control of the house of representatives or the senate.

Some believe voters’ opinions of Mr Trump could prevent candidates from surpassing his own level of popularity if he becomes embroiled in tight congressional contests.

The president will have done nothing to help the campaign in Ohio after he mistakenly endorsed a candidate who is not even on the ballot.

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In now-deleted tweet, he offered support to Steve Stivers, a Republican congressman who already represents Ohio’s 15th district.

“[Mr Stivers] is a great Congressman who is tough on crime & borders & an inspiration to our Military & Vets,” Mr Trump wrote.

“Get out and vote for Steve on Aug 7th. He has my full & total endorsement.”

Mr Trump is seemingly casting himself as the star of the midterms, inserting himself into hotly-contested primaries, headlining rallies in pivotal swing states and increasing his fundraising efforts for Republicans.

Last week, he agreed to donate a portion of his re-election fund to 100 GOP candidates running in competitive House and Senate races.

He's expected to be even more aggressive in his canvassing once autumn arrives.

White House officials say Mr Trump is already reserving time on his schedule for midterm campaigning and fundraising likely to surpass efforts 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 mobilising Trump loyalists is enough to win in close congressional districts or if victory depends on winning over independents and those turned off by the president’s turbulent first term.

“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,” Mr Cardenas added.

“You can't win a presidential election if you're a Republican without winning Florida.”