In a trip to western Pennsylvania in January, President Donald Trump promoted the recently passed Republican tax plan as a boon to workers and businesses in a region that helped to propel him to the White House.

The president trumpeted "tremendous tax relief" for families. Trump brought an employee of Pittsburgh-area H&K Equipment to the stage to talk about how he would spend his tax savings. Trump also listed the major Pennsylvania employers who planned to give employees bonuses after a massive reductions in their tax burdens.

"When we began our push for tax cuts, I promised that our bill would result in more jobs, higher wages and tremendous relief for middle-class families, and that is exactly what we have delivered," Trump said at the time. "There's only one thing — even I never knew how big it would be. It's much bigger — and you see it — than anybody anticipated. We kept our promise."

Trump's visit came partly as an effort to boost Republican state Rep. Rick Saccone, who is running in a tight House special election in southwestern Pennsylvania's 18th District. On Saturday, the president is expected to head back there, in a new bid to promote Saccone's candidacy.

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While most voters who spoke to CNBC in the red district this week at least modestly supported the tax plan or said they knew someone who benefited from it, the tax overhaul has not featured as prominently as an election issue as the president may have hoped in January.

Ahead of November's battle for control of Congress, the Pennsylvania race is seen as a crucial gauge of Democrats' ability to win House seats in red areas like the 18th District, which Trump won by 20 points in 2016. A strong U.S. economy and the tax plan will be a key selling point for Republican candidates in this year's midterms.

Trump may try to refresh voters' minds about the tax plan when he makes another visit to western Pennsylvania Saturday evening, just days before Tuesday's election. The law slashed tax rates for businesses and moderately reduced the tax burden on most individuals.

Democratic leaders in Congress have cast it as a giveaway to the rich and corporations at the expense of the working class. Republicans have cited corporate announcements of one-time bonuses or capital investment as evidence that the plan worked, although stock buybacks have also shot up since the tax overhaul became law in December.