WASHINGTON -- The core political challenges facing Democrats are not the rise of those who proudly call themselves democratic socialists and the danger that Republicans will succeed in red-baiting the entire party.

Instead, Democrats face formidable coalition management problems because they now provide a home to millions of voters (and scores of elected officials) who in earlier times might well have been liberal Republicans.

Democratic leaders -- and presidential candidates -- must find ways to cope with an alliance that spans not only their own long-standing left and center-left factions, but also many moderate voters who despise President Trump but have not been Democrats before.

The fractious dustup in the House Democratic caucus last week over how often its vulnerable members should be able to vote with the GOP is another reminder of the difficulty of holding a big-tent party together.

Here's the key point: The 2018 elections did not make the Democrats a more left-wing party. It had the opposite effect. A large share of the new Democrats in the House hail from districts -- a lot of them suburban -- that in the past would have happily elected Republicans with moderate-to-progressive inclinations.

Such Republicans, once a substantial minority in the party, are a virtually extinct species. The rightward shift of the GOP began before Trump's rise, and his extremism has, in turn, led to the defeat of even moderate conservatives. The survivors (with occasional brave exceptions) generally moved his way, fearing defeat in primaries.

This has had a peculiar effect on our politics: Many of the most important policy debates are no longer between the two parties; they are being carried out almost entirely inside the Democratic Party.

Because of the hold right-wing ideologues and extractive industries have on their party, Republican politicians are under great pressure to deny that climate change has human causes. Therefore, Democrats tussle over whether carbon taxes or the provisions of an ambitious Green New Deal are the best way to mitigate an impending catastrophe.

Republicans don't even support the advances in health insurance coverage brought about by Obamacare. Therefore, the question of how to achieve universal coverage is left to the Democrats. They quarrel about the relative merits of a Medicare-for-all system or incremental steps building on the Affordable Care Act.

My hunch (and hope) is that those preferring single-payer health care will come to see that, if their goal is ever to be realized, it will not pass in one big bang. It will be achieved in steps that would leave a private insurance system in place for some time. But in the short term, there's a lot of shouting.

Republicans are for cutting taxes, and then cutting them again, always in ways that tilt the code further toward the wealthy and corporate interests. They do this by ignoring the deficits they are increasing, except for mouthing vague bromides about "reforming entitlements." This sticks Democrats with the burden of raising enough money to pay for both existing and new programs.

These issues are far bigger than the strife roiling the House, though it matters, too. A significant number of moderate Democrats believe their political survival requires them to vote for nuisance amendments with appeal in their districts put forward by Republicans.

House Speaker Nancy Pelosi and progressive members of her caucus want the moderates to vote "no" on all these last-minute motions, as Republicans generally did when they were in the majority. Pelosi understandably fears that throwing votes the GOP's way increases their ability to disrupt legislation.

Yet many of the politically moderate freshmen, backed by other members of the leadership, naturally worry about protecting their fragile electoral margins. They thus argue that they should be able to vote with the Republicans fairly freely.

House Democrats will eventually resolve this problem by working out a better disciplined system of granting a limited number of "free passes" on especially tough votes while preventing wholesale defections. Modest procedural changes (for example, by allowing at least an hour between the introduction of an opposition amendment and the vote on it) might also ease the internal strains.

But even if they work through this kerfuffle, the larger challenge remains. Democrats need to figure out how to make genuine progress on the issues that rightly engage their party's left -- for starters, health care, climate change, and rising economic inequality -- in ways that allow their new constituency of virtual liberal Republicans to join the effort. The party's presidential candidates should focus more of their energy on explaining how they'll pull this off.

(c) 2019, Washington Post Writers Group