The steady path toward today’s partisan polarization at the Capitol is etched in the history of tax bills over more than half a century.

If President Donald Trump is able to pull off his uphill drive to join most of his predecessors since World War II in securing a significant tax cut, it’s very possible he’ll do so exclusively with the votes of congressional Republicans.

After months of jawboning, trial balloons and practiced positioning, the substantive debate is about to shift rapidly into top gear — assuming House Republicans keep to their plans for a Wednesday unveiling of their actual, detailed opening bid for tax reductions totaling $1.5 trillion in the coming decade. GOP leaders on the House and Senate sound committed to trying to accomplish what Trump is demanding, which is to get a bill on his desk no later than the end of the seven weeks of scheduled legislating for this year.

It’s a reasonable bet that not a single Democratic senator or House member will end up lending a hand — at least not unless the GOP demonstrates clearly it has gotten beyond its own deep internal anxieties over the details and has the capacity to pass the package on its own.

Enacting a deep tax cut entirely on party lines, like so much else about the Trump presidency, would be unique in the modern era.

Passing contentious policy changes and sprawling spending packages with few crossover votes, or with only minimal support from the minority party, has grown more and more normalized — to the point where that approach has become almost the default expectation in recent years, whether Democrats or Republicans wielded the balance of power.

But sweeping revenue bills, or at least those that have made it into law, have remained different.

Reaching across the aisle

Each of the half-dozen major tax cuts pushed to enactment by the presidents of the second half of the 20th century — Dwight D. Eisenhower in 1954, Lyndon B. Johnson completing John F. Kennedy’s efforts in 1964, Richard Nixon in 1971, Jimmy Carter in 1978, Ronald Reagan in 1981 and Bill Clinton in 1997 — had one thing in common: Most Democrats and most Republicans, in both the House and the Senate, voted “yes” on final passage. In all but a couple of cases the bipartisan majorities were lopsided.

The same was also true of the 1986 statute, the revenue-neutral overhaul and simplification of the IRS rulebook that has stood for the subsequent three decades as the Holy Grail of “tax reform” that Republicans have wanted to emulate — now in conjunction with one of the deepest tax cuts in American history. The final deal won the support of 65 percent of the Republicans and 70 percent of the Democrats in the House, along with three-quarters of the senators from each party.

Of course, the partisan demographics at the Capitol were fundamentally different throughout those decades, because plenty of — mostly Southern — Democrats were more fiscally and socially conservative than a solid clutch of Republicans mostly from the Northeast. That dynamic started fading during the George W. Bush presidency, and, as a result, his enormous 2001 tax cut won the support of a comparatively small roster of 40 Democrats across the Hill and that number dwindled to just nine for his more modest cut two years later.

Since the end of the last decade, there has been no Democrat with a voting record to the right of any Republican, which is one reason why just three lawmakers who voted for a Bush tax cut remain in office: Sen. Dianne Feinstein of California and Rep. Collin C. Peterson of Minnesota, who backed Bush’s first cut but not the second, and iconoclastic Rep. David Scott of Georgia, who voted for the 2003 bill.

Of that group, Trump’s only realistic shot is Peterson, because he’s on course to extend his three-year run as the Democrat most likely to go against the grain on votes that fall mainly along party lines. However, he joined the unified House Democratic Caucus that voted last week against adopting the budget resolution — the principal purpose of which was to include language permitting the coming tax bill to get through Congress without any Democratic votes.

Instead, 20 Republicans opposed the budget, mostly to signal their displeasure that the emerging tax plan might limit the federal deduction for payments of state and local taxes and reduce the tax benefits of saving for retirement in 401(k) accounts.

Possible complications

Those are just the two most high-profile proposals that have created the earliest visible internal GOP consternation. Dozens more lurk just beneath the surface, but they’re bound to burst forth in coming days. Such a comprehensive and once-in-a-generation bill would affect almost every niche of the national economy, and so is guaranteed to stand as the most heavily lobbied measure in years.

There is no statutory deadline, but the extraordinarily tight timetable Trump is pressing — so he might claim one landmark legislative win in his presidency’s first year — only magnifies the opportunity for the entire enterprise to choke on the coming web of rapid-fire and contradictory advocacy demands.

The Republicans are betting that the inverse will be true, and that mandating a brief but intense period for debate will maximize the odds they remain sufficiently cohesive while minimizing the chances that K Street could nitpick the package to death during the time it spends in the sunshine.

That dynamic is not made easier by Trump’s developing of a penchant for encouraging Congress to work its will and then weighing in at crucial junctures in fragile backroom negotiations, bluntly and on Twitter.

The Democrats, for their part, have so far relied on the blanket allegation that whatever Trump and his congressional allies settle on is sure to live up to the stereotypical description of a Republican tax cut: Disproportionate benefits for the richest Americans masquerading as overdue help to middle-class families and a turbocharge of the economic engine.

The minority party’s leaders  have been encouraging potential mavericks to keep their powder dry as long as possible — with the argument that there’s no political benefit to becoming early collaborators in a tax cut effort that threatens to implode under the weight of Republican dissent.

No Democrat voted for the budget in the Senate, either. Adoption of that document means the tax bill is immune to filibuster and can pass the Senate with a simple majority. But that, as with the drive to replace the 2010 health care law, means any more than two GOP strays will kill the tax legislation and deny Trump the victory he so craves — so long as Democrats stay unified.

If there’s going to be any of the sort of bipartisanship that’s been a hallmark of all the other successful tax cuts, the most viable scenario is this: The Republicans push their bill through the House with a survivable number of defections, and then GOP senators show even more unity by marshaling a majority of 51 (out of 52) even without Vice President Mike Pence breaking a tie.

Only then, under this storyline, might a handful of the Democratic senators in the most danger of losing bids for re-election next year be emboldened to go along. The likeliest four, all of them running in states Trump carried easily in 2016, are already atop the leader boards among senators who have both backed Trump the most often and strayed from the party line most frequently this year: Joe Manchin of West Virginia, Heidi Heitkamp of North Dakota, Joe Donnelly of Indiana and Claire McCaskill of Missouri.