(CNN) Since Ronald Reagan was elected president in 1980, the Republican coalition has revolved around a delicate but durable balancing act. The ongoing drive to complete the GOP's tax bill will test whether that balance can endure under the heightened stress Donald Trump's tumultuous presidency is imposing on it.

Over roughly the past 40 years, the GOP has pursued a two-track approach toward building its electoral coalition. First, Republicans have consistently targeted the cultural conservatism -- and often the racial resentments -- of the blue-collar, evangelical and rural whites who are most uneasy about the social and demographic changes remaking American life. Simultaneously, in the name of promoting capitalism and free enterprise, the party has targeted its economic agenda primarily at the priorities of the wealthiest earners and business.

To a far greater extent than his iconoclastic rhetoric as a candidate suggested, Trump in office has closely followed that twin path. Yet he has complicated this balance nonetheless by intensifying the pressure on each side of the fulcrum.

JUST WATCHED Trump wants this tax cut. It was a disaster in Kansas Replay More Videos ... MUST WATCH Trump wants this tax cut. It was a disaster in Kansas 04:07

Compared with earlier Republican leaders, Trump has far more explicitly appealed to racial resentments -- from his campaign trail attacks on Judge Gonzalo Curiel to his defense of the white supremacists marching in Charlottesville, Virginia, to his tweets last week describing LaVar Ball, the African-American father of one of the UCLA basketball players accused of shoplifting in China, as an "ungrateful fool."

From the other direction, Trump and Republican congressional leaders have advanced an economic agenda that tilts more lopsidedly toward the top than earlier generations of GOP policy-making. Past Republican economic plans -- like the tax cuts passed under Presidents Reagan and George W. Bush -- might have channeled more of their benefits toward the wealthy than to the middle -- and to working-class voters drawn to the party largely on cultural grounds. But the congressional GOP, with Trump's acquiescence, now has advanced proposals that actually impose costs on many middle- and upper-middle-class households to fund its benefits for the wealthy.

Read More