PALM SPRINGS, Calif.—One of the Republican Party's biggest donor groups is dramatically ramping up its political spending – and putting the recently passed tax plan front and center – as the GOP fights to preserve its congressional majorities in this year's hotly contested midterm elections.

The network of advocacy groups tied to billionaire industrialists Charles and pledged to spend close to $400 million on campaign contributions and policy initiatives in the lead-up to the vote in November, a 60 percent jump in spending from the 2016 election cycle, officials said. One of the hallmarks of that effort is a fresh influx of support for the Republican tax plan, with up to $20 million devoted to selling its benefits to voters this year.

The tax pitch will begin in a few weeks, just as companies begin to implement the new law and workers see changes in their paychecks. The Trump administration estimated that 90 percent would enjoy an increase in wages, and Republicans have touted recent announcements of bonuses and pay raises from businesses across a wide swath of industries.

"Part of our job is to make sure those benefits are burning through the clutter and the normal give-and-take of American politics," Tim Phillips, president of Americans for Prosperity, said during a private meeting of donors and supporters Saturday in Palm Springs.

The Koch network was instrumental in helping Republicans score their biggest legislative victory last year. The group spent $20 million last year to encourage passage of the tax bill, running TV and digital campaigns, holding 100 town halls across 36 states and attending high-profile meetings at the White House to shape the final legislation.

Officials said the new sales campaign will adopt a similar approach, delivering a positive message that Republicans hope will resonate with an increasingly jaded constituency. Although polls generally show the bulk of Americans oppose the tax plan or do not believe they will benefit from it, recent surveys also indicate support is increasing.

"There is a healthy skepticism among a majority of Americans about politics in this country," Phillips said. "We do think the bar is a bit higher."