President Donald Trump flexed his kingmaker ability in Republican primaries across the country this year, deciding whether to sink or help candidates in a party that has increasingly cast itself in his image.

The president may soon realize his endorsements carry less weight in midterm election swing districts.

So far, Trump has supported few GOP candidates in the most competitive House districts that will determine which party controls the chamber after November's elections. He picked up his endorsement pace in recent days with stops in battleground Minnesota, Kansas and Iowa districts, which he will follow with rallies in Pennsylvania, Ohio and Kentucky.

But early signs indicate Trump's backing holds less sway in some of the more ideologically divided districts that Republicans need to win to keep the House. While Trump's support should boost Republicans trying to unseat Senate Democrats in staunchly pro-Trump states, he may not help much in the battle for the House, where opposition to the president and his policies has defined many races.

Polling suggests Trump's tweeted endorsement of candidates in two key GOP-held districts has failed to help. In western Pennsylvania's 17th District, where incumbent GOP Rep. Keith Rothfus and Democratic Rep. Conor Lamb face off due to redistricting, the president's support for the Republican has not moved the needle, according to a Monmouth University poll released Wednesday.

Only 11 percent of likely voters there say the endorsement makes them more likely to support Rothfus, while 28 percent say it makes them less likely and 61 percent say it has no effect. Still, just 33 percent of likely voters have heard about Trump's endorsement.

"It's probably good news for Rothfus that few voters are aware of Trump's endorsement because it really doesn't help him," Patrick Murray, director of the Monmouth University Polling Institute, said in a statement. The poll showed a double-digit lead for Lamb in three different voter turnout models.

Trump also endorsed Jay Webber, a New Jersey state assemblyman competing with Democratic former prosecutor Mikie Sherrill to replace retiring GOP Rep. Rodney Frelinghuysen in the state's 11th District. A Monmouth poll released Tuesday found a lead for Sherrill, but only 31 percent of likely voters said they heard about the president's support for the Republican.

Regardless of whether they knew about Trump's endorsement, only 15 percent said it would make them more likely to vote for Webber. Twenty-eight percent responded that it would make them less likely to back the Republican, while 56 percent said it would make no difference.

"It's no surprise that Webber has not been trumpeting the president's endorsement when you look at these poll numbers. Trump does not really help even though this is a Republican district," Murray said in a statement along with that poll release.

Both districts have a slight Republican lean. The Pennsylvania and New Jersey seats rate as "R+3" seats, according to Cook Political Report's Partisan Voter Index. It measures how districts vote relative to the nation as a whole in recent presidential elections.

Monmouth has not yet polled voters about a Trump endorsement in other swing districts, and CNBC could not find House surveys that asked a similar question.

Trump only recently started backing more candidates in races considered highly competitive, including Republican Jim Hagedorn in Minnesota's Democratic-held 1st District and GOP Rep. David Young in Iowa's 3rd District. Nonpartisan forecasters consider both contests toss-ups. The Minnesota and Iowa seats have PVI ratings of "R+5" and "R+1," respectively.

Of course, polls in two districts do not capture how Trump's endorsements will influence other races in distinct areas. But they suggest that Trump may have some tough decisions to make about whom to endorse and hold rallies for as he heads out on an arduous campaign schedule in the month before the elections.

The composition of the House has big stakes for him, as Democratic control could make it harder for him to pass his priorities and lead to congressional investigations of his administration.

The White House, for its part, thinks Republicans need to tie themselves more closely to Trump if they want to hold a House majority, according to The New York Times. A White House spokeswoman did not immediately respond to a request to comment on whether the administration thinks Trump could hurt some Republican House candidates.

In addition, nothing indicates Trump's endorsement of Senate or House candidates in areas he won overwhelmingly hurts them. The president's support appears particularly important in states such as Missouri, Indiana, North Dakota and West Virginia, which he carried by roughly 20 points or more as he won the presidency.

Vulnerable Democratic Senate incumbents in those states such as Heidi Heitkamp of North Dakota and Joe Donnelly of Indiana have touted their efforts to work with Trump as they try to hold their seats. Recent polls have largely showed Heitkamp trailing GOP Rep. Kevin Cramer, while Donnelly has a slight lead over Republican former state lawmaker Mike Braun.

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