The Democratic presidential primary offers a clear choice. On one side there's Hillary Clinton, a battle-scarred pragmatist who has learned from long experience that governing is difficult and big dreams lead only to big losses. She's been in the White House, she's served in the Senate, and she has no illusions about how the sausage gets made. On the other side there's Bernie Sanders, an idealist with a sweeping vision, unafraid to call himself a democratic socialist, willing to advocate for what he thinks is right no matter who says it's impractical.

We saw the contrast in their last debate, particularly on the issue of health care. Sanders advocates a single-payer insurance system similar to what is in place in every other industrialized country, and he appealed to basic progressive values and cited Democratic heroes: "Frank Delano Roosevelt, Harry Truman, do you know what they believed in?" he said. "They believed that health care should be available to all of our people." Clinton countered that there's no way such a thing could pass Congress, observing that Democrats couldn't even get the votes to add a public option to the Affordable Care Act, let alone remake the whole system as one big public option. "So, what I'm saying is really simple," she said. "This has been the fight of the Democratic Party for decades. We have the Affordable Care Act. Let's make it work."

It was highly illuminating. But perhaps the difference between the two candidates isn't as wide as it seems. To a degree, they're both campaigning as idealists, insofar as neither one has articulated anything resembling a plan to deal with the single greatest challenge the next Democrat will face: a Republican Congress.

Because if one of them does win November's election, that's what they'll face. Even in the best of circumstances — a terrific election for Democrats, with the new president trailing long coattails — one or both houses will still remain in GOP control.

Let's start with the Senate. Since it's six years after the Republican sweep of 2010, the people who won that year are up for re-election, which means that Democrats only have to defend 10 seats, while Republicans have to defend 24. But most of those are in conservative states where there won't be much of a contest. Democrats are likely to pick up few seats, but they'll need four seats to reach a 50-50 tie, which a Democratic vice-president would break. In order for this to happen the Democrats would have to win all the competitive races — most election analysts rate as few as five races as truly in play (see here or here).

But let's say the Democrats do take the Senate. No one thinks they have any chance of taking the House, where Republicans currently hold a 58-seat advantage. And whether Republicans hold one house or two, they can stop any legislation the Democratic president wants in its tracks.

And they will, have no doubt. When Barack Obama was inaugurated in 2009, Republicans decided on a strategy of maximal opposition, and from their perspective it was extremely effective. They won back both houses of Congress, and after a brief period at the beginning of the Obama presidency, stopped any ambitious legislation from passing. Apart from a few bipartisan bills here and there and measures to keep the government open, Congress hasn't done any lawmaking in years.

Are Republicans any more likely to change their strategy for President Clinton or President Sanders? Keep in mind that the Republican caucus is substantially more conservative now than it was in 2009, with moderates purged, tea partiers elected, and everyone scared of a primary challenge from the right. So the Democratic president won't have a lot of people from the other party eager to get any legislative work done.

How do Clinton and Sanders say they'll deal with that? They don't really explain. Sanders makes it sound as if the "revolution" he wants to lead will steamroll over any opposition, but unless he plans to remake the way a bill becomes a law, it's hard to see how. And while Clinton's ideas aren't quite as sweeping, she still has many substantial things on her agenda — increasing the minimum wage, boosting renewable energy, comprehensive immigration reform, paid family leave, and many more. Why should we think Republicans will agree to any of it, when they're not only ideologically opposed to those things but will want to deprive her of any legislative victories? Most Republican members of Congress come from conservative states and districts, so they won't pay a price for standing in the way of Clinton's agenda.

To make matters worse, there will only be so much a new president can do outside of legislation. President Obama has been quite frank that on certain issues like immigration, in the face of congressional intransigence he's trying to do everything he can, within the limits of his executive authority, to make progress without congressional approval. The problem is that there are limits to that authority (and Republicans have challenged many of those actions in the courts). Right now there are people in the executive branch working hard to come up with more actions that can be taken using the president's powers, and by a year from now, they will have unearthed many of them. If Clinton or Sanders takes office at that point, how many more executive orders to advance progressive goals could they find?

I realize this is a depressing picture, one of four or eight more years of gridlock, with a hamstrung president who can do little more than preserve the policy legacy of the Obama administration. That's a worthy enough goal, but it isn't the kind of thing that gets your face chiseled on a mountain.