At least 41 Democratic senators have publicly committed to filibuster President Trump’s Supreme Court nominee, Neil Gorsuch, leading to a probable showdown with Republican Senate Majority Leader Mitch McConnell.

The filibuster might seem like payback for Democrats after Republicans refused to consider the nomination of then-President Obama’s Supreme Court nominee, Merrick Garland, for 293 days starting last year. Unlike Republicans last year, however, Democrats don’t have all that much power. They aren’t in the majority — and McConnell has strongly hinted that he could seek to eliminate the filibuster for Supreme Court picks if Gorsuch can’t get 60 votes. Across a variety of surveys, moreover, a plurality of voters think the Senate should confirm Gorsuch, although a fair number of voters don’t have an opinion either way. Therefore, Democrats’ political endgame is unclear.

Gorsuch is quite unpopular with liberal voters, however: By a 61-15 margin, they oppose his confirmation, according to a YouGov poll last week. Thus, the planned filibuster may simply be a sign of the liberal base’s increasing influence over the Democratic coalition. The share of Democrats who identify as liberal has steadily increased over the past 10 years. According to the recently released Cooperative Congressional Election Study, 53 percent of Democratic voters identified as liberal last year. Until recently, it was rare to find surveys that showed liberals made up a majority of the party.

But to some extent, that 53 percent figure understates the case. The CCES also asked voters about whether they’d engaged in a variety of political activities, including donating to a candidate, attending a political meeting, working on behalf of a campaign or putting up a political sign. Among Democrats who’d done at least one of those things — a group I’ll call “politically active Democrats” — 69 percent identified as liberal. These were some of the voters who helped propel Bernie Sanders to almost two dozen primary and caucus victories last year.

Oftentimes these liberals are found in states where you might not necessarily expect them — such as in the Mountain West, which was a strong region for Sanders last year. According to a regression analysis conducted on the CCES data, the proportion of politically active Democrats who identify as liberal is larger in states where candidate Trump fared poorly. But controlling for that, it’s also larger in states that have more white voters, and more college-educated voters. And it’s larger in the West than in the other political regions of the country. In the table below, I’ve estimated the share of politically active Democrats in each state who identify as liberal. Since the sample sizes for some states are small, the estimates are based on a blend of the raw polling data from the CCES and the regression model I described above.

SHARE OF POLITICALLY ACTIVE DEMOCRATS THAT IDENTIFY AS LIBERAL STATE DEM SENATORS UP FOR RE-ELECTION POLL-BASED ESTIMATE MODEL-BASED ESTIMATE BLENDED ESTIMATE D.C. 87% 77% 84% Idaho 92 74 82 Utah 85 74 80 Washington Cantwell 78 76 78 Minnesota Klobuchar 78 74 78 Oregon 77 78 77 New Hampshire 81 71 76 Vermont Sanders 78 75 76 Montana Tester 73 77 76 Alaska 84 71 75 New Mexico Heinrich 76 73 75 Maine King 78 69 74 Arizona 75 70 74 Massachusetts Warren 75 72 74 Connecticut Murphy 75 72 74 Rhode Island Whitehouse 74 73 73 Virginia Kaine 73 68 72 California Feinstein 72 74 72 Michigan Stabenow 73 69 72 Indiana Donnelly 73 68 72 Wyoming 83 67 72 New York Gillibrand 72 71 72 Iowa 72 71 71 Illinois 71 70 71 South Dakota 82 67 71 Colorado 69 75 70 Nebraska 73 67 70 Arkansas 78 59 70 Florida Nelson 69 66 69 Nevada 68 69 69 Tennessee 70 63 68 Pennsylvania Casey 68 70 68 Delaware Carper 63 72 68 Wisconsin Baldwin 67 72 68 Ohio Brown 68 68 68 Kansas 64 70 66 Mississippi 73 54 65 North Carolina 64 66 65 Texas 64 63 64 New Jersey Menendez 63 67 63 Oklahoma 63 61 63 Louisiana 61 66 63 North Dakota Heitkamp 51 66 61 Kentucky 60 64 61 Hawaii Hirono 43 71 61 Missouri McCaskill 60 65 61 Alabama 61 57 59 Maryland Cardin 58 64 59 West Virginia Manchin 49 67 57 South Carolina 51 59 54 Georgia 49 61 51 Where is the Democratic base most liberal? Source: Cooperative Congressional Election Study

It’s not surprising that Washington, Oregon and Vermont are places where the liberal wing of the Democratic base dominates. But Idaho, where I estimate that 82 percent of politically active Democrats identify as liberal, and Utah, where I estimate that 80 percent do, also rate near the top. It’s not that Idaho and Utah are blue states, obviously; they’re among the most Republican in the country. Nonetheless — perhaps because a lot of moderate voters identify with the GOP in these states — the few Democrats that remain are overwhelmingly liberal.

The same phenomenon holds in Montana, where I estimate that 76 percent of politically active Democrats are liberal. That may help to explain why Sen. Jon Tester of Montana says he will vote against Gorsuch, even though he faces a tough general election campaign next year. Whether or not Democrats would issue a primary challenge to Tester, who has generally sided with the party on key votes, is questionable. Nonetheless, he’ll be relying on his base for money, volunteers and a high turnout on Election Day. In Montana, the conservatives are conservative — but the Democratic base is fairly liberal also.

By contrast, Democratic Sens. Heidi Heitkamp of North Dakota and Joe Manchin of West Virginia, who will vote to confirm Gorsuch, are on somewhat safer ground. Some 61 percent of politically active Democrats identify as liberal in North Dakota, while 57 percent do in West Virginia, according to this estimate. Those figures are almost certainly higher than they would have been a few years ago. But Heitkamp and Manchin probably face more risk from the general election than from a loss of support among their base.

Nor is the Democratic base all that liberal in the Mid-Atlantic region, including states such as Maryland, Delaware and New Jersey. Instead, even the party activists in these states can have a moderate, pro-establishment tilt. That may explain why senators such as Chris Coons of Delaware and Robert Menendez of New Jersey were slow to announce their positions on Gorsuch before eventually deciding to oppose him.