During the course of his presidential campaign, Bernie Sanders developed an enviable donor list. That’s why Harry Reid went to Sanders and asked for his help in raising money and other support for Democratic Senate candidates all over the country. On Monday, Sanders came through by sending out an email with the title: Winning the Senate. The email explains:

“I want to be clear: It is very important that our movement holds public officials accountable. The Democratic Party passed an extremely progressive agenda at the convention. Our job is to make sure that platform is implemented. That will not happen without Democratic control of the Senate.”

However, winning the Senate is clearly a secondary goal for Sanders. His first priority is electing a certain kind of Democrat. Former governors Maggie Hassan and Ted Strickland of, respectively, New Hampshire and Ohio are worthy of support. Neither Reps. Patrick Murphy nor Alan Grayson of Florida pass that test. Katie McGinty of Pennsylvania and former Nevada Attorney General Catherine Cortez Masto get an endorsement, but the same cannot be said of Patty Judge of Iowa or Deborah Ross of North Carolina.

Sanders could have told his supporters to back Ann Kirkpatrick in Arizona and Tammy Duckworth in Illinois and Evan Bayh in Indiana, but he didn’t.

The outlines of some sort of logic are evident. Grayson may be progressive on a lot of issues but he’s also morally compromised. Evan Bayh, Patrick Murphy and Ann Kirkpatrick are very centrist Democrats. Tammy Duckworth, way back in the mists of time (2006), won a contentious primary against Christine Cegelis, a progressive so pure that she refused to endorse Duckworth’s bid in the general election to take over Henry Hyde’s vacated House seat. Patty Judge, the 46th Lieutenant Governor and once Secretary of Agriculture for Iowa, just defeated a Wellstonian progressive who campaigned in his Prius touting the excellent example of FDR’s righthand man, Iowan Harry Hopkins, and their record creating the Works Progress Administration (WPA).

On the other hand, Sanders did give a shout-out to Katie McGinty even though most Sanders supporters in the Keystone State lined up for her primary opponent, Joe Sestak.

It makes sense for Sanders to apply some kind of standard for his support, even if it’s not all that clear what the standard is. If he were to just endorse every Democrat on the block, regardless of where they stand or have stood, then he’d be no different than DSCC chairman Jon Tester or any standard partisan yellow dog Democrat. His campaign was not just about beating Republicans. It was also about reforming the Democratic Party, which he might have done if he had chosen to remain a party member for more than 72 hours after losing his bid to lead it.

Yet, the email funding solicitation says that it’s about “winning the Senate,” and that enacting a progressive agenda “will not happen without Democratic control of the Senate.”

But, guess what?

It’s not likely that there will be a Democratic Senate if the Dems don’t win with at least some of the candidates that Sanders is pointedly not supporting. Maybe Tammy Duckworth doesn’t need his help and Evan Bayh doesn’t want it, but Patty Judge and Deborah Ross are in tough campaigns where they’re going to be badly outspent.

I mention all this not so much to pick on Sanders, because I recognize and respect the imperatives he has as a leader of progressive-minded people. The reason I find it instructive is because it points out rather starkly the intersection of aspirational and pragmatic politics. There really is a “Professional Left” (I know, I used to be employed by it) whose job isn’t to “win the Senate” at all, but to push for issues and empower progressives over squishes within the Democratic Party. In practice, this means sending message after message to left-wing Democrats that will outrage them and get them to give money, sign petitions, and become members of your organization. Any effort that fails in those three respects, will not be repeated.

But, behind these efforts, and before they become jaundiced and curdled, is real idealism and a quite rational belief that some problems in America can’t wait, are not going to be fixed incrementally, and require systemic and radical change.

Set against these idealists, now and always, are equally (perhaps more) rational pragmatists who will point out uncomfortable truths like the fact that some progress can be made on Bernie Sanders’s agenda with a Democratic Senate, even if that Senate majority is only made by the election of Evan Bayh, and that no legislative progress can made with a Republican Senate.

This pragmatic truth is unassailable enough that it’s the exact pitch that Sanders made to his supporters in his email.

The Sanders delegates helped craft the most progressive Democratic platform in history, and they won’t see any of it come to anything unless the Democrats “win the Senate.” (The Dems would need to win the House, too, but that’s another email, right?).

So, what’s the error that the idealists make?

The error is to misdiagnose the problem in a way that makes them blind to how progress actually happens in our system.

The last eight years offer all the evidence you need to see this. President Obama swept large Democratic majorities into Congress which he used in 2009 and 2010 to pass a flurry of legislation (see a list here). Quibble all you want with the details, but more got done on a progressive agenda in those two years than had been accomplished in eight years of Bill Clinton’s presidency or in all the years since Lyndon Johnson left the West Wing. But this was also the peak of power for the fiscally and (often) socially conservative Blue Dogs. When the Blue Dogs were wiped out in the elections of 2010, it took the Democrats’ control of Congress away and all legislative progress came to a screeching halt. Instead, we debate austerity and whether we can even keep the government open and pay our bills on time.

Conservative Democrats may have vetoed valuable amendments or prevented them from even being seriously discussed, and they may have introduced garbage into the legislative process that did substantive and political damage to the Democratic Party and the left. Some of them may have lent aid and comfort to the Republicans’ most outrageous and cynical gambits. But what determined whether progress was made was which party controlled Congress.

It’s a second order of concern how that majority party is organized. It certainly mattered that Max Baucus had a huge role to play in crafting the Affordable Care Act. It mattered that Joe Lieberman had enough power to effectively rule out an expansion of Medicare. It mattered that a budget hawk like Kent Conrad was chairing the Budget Committee and that there were a lot of Democrats in Congress too reticent about offending Wall Street to enact the strongest possible post-Great Recession reforms. From a progressive point of view, getting better Democrats and getting them into the right places to exert meaningful power are key goals. Every bad Democrat is a potential bottleneck or worse.

But getting better Democrats is still the second order of concern, and it’s dwarfed by the first.

If you need a little more evidence for this, take a look at a guy like J. Lister Hill. When he retired as an Alabama senator in 1969, he had been in Congress for forty-five years and in the Senate for thirty. Yes, he signed the Southern Manifesto, but he was also strongly supported by Labor. He was a key backer of the Tennessee Valley Authority who was responsible for great achievements in rural electrification, backed federal control of offshore drilling, and was a champion of the physically and mentally disabled. Much of this, he was able to accomplish as the longtime chairman of the Senate Labor and Public Welfare Committee. Like most Southern Democrats of his time, he almost never faced serious Republican opposition and so rose in seniority. When LBJ passed his Great Society legislation, Congress was littered with southern (often pro-segregation) chairmen. What mattered more was that the 89th Congress had 67-68 Democrats and 32-33 Republicans in its Senate and began with a 295-140 Democratic majority in the House.

If you want to know how this worked in practice, the U.S. Senate site has an oral history provided by Stewart E. McClure, who served as Chief Clerk of Sen. Lister Hill’s Committee on Labor, Education, and Public Welfare. He explains the “avalanche of domestic legislation that the committee handled during the Great Society and offers candid assessments of the internal politics and stresses of committee life during those years.”

Lister Hill was a populist, and liberal for his time and place, but he was still officially a segregationist Democrat. And, yet, he was an important supporter of The New Deal and a key architect of the Great Society, just as Max Baucus was a very corporate and centrist Democrat who was a key architect of Obamacare.

We need idealists and pragmatists, and some battles require the long view. The problem only arises when one group begins to see the other as the primary problem rather than respecting the key role that each respectively plays in achieving progress.

If Hillary Clinton is going to accomplish anything legislatively in office, she’ll do much better with a Congress like Obama enjoyed in his first two years than the ones he’s been grappling with during the last six.

So, yes, winning the Senate takes precedence over purity. Sanders admits as much, in his own way, but then he can only take that so far and keep his credibility as a progressive champion.