FOR those Democrats, brimming with anti-Trump zeal, who think now is the time to cleanse themselves of ideological impurity, Claire McCaskill has a message. “I want people to understand, particularly in the progressive wing of my party,” said the senator from Missouri, who is fighting a tough battle for re-election in November, “that it’s not just a question of whether I am perfect for them on all the issues. It is that, if I lose, a Republican will take my place who’d be anathema to them on every issue.”

Speaking on the fringe of a campaign event in St Louis, Missouri’s second city, Ms McCaskill had had a trying week. Her state elected President Donald Trump by a 19-point margin, mainly, as everywhere, because he promised to keep out foreigners. Yet her party, egged on by its progressives, had just shut down the federal government in a botched bid to protect a multitude of illegal immigrants. Ms McCaskill, a tough operator with a prosecutor’s mind and a folksy manner, joined four other moderate Democrats in voting against the shutdown. She was also, though flu-ridden, involved in negotiating an agreement to end it. Naturally, this won her no favours from her Missourian opponents, who consider her a liberal cuckoo in a Republican nest, or from ambitious progressives such as Kamala Harris and Elizabeth Warren, who decried the shutdown’s swift conclusion. At least she is used to it. “No matter what I decide on any issue, half the people are mad at me,” she said, rather optimistically.

Her difficulty points to a paradox in American politics. While both parties have moved further to the extremes in recent years, making it increasingly hard for centrist politicians to operate, the tightness of the contest means those same politicians tend to decide which party holds sway. In other words, moderates have never been more marginalised or more important. The hyper-partisanship around Mr Trump’s presidency has increased that tension further, especially in the Senate, where ten Democrats are up for re-election in states the president won. Most are moderates, including five in states he won by crushing margins: Indiana, Missouri, Montana, North Dakota and West Virginia.

That makes their task daunting. Mr Trump’s victories represented, among other things, a rightward turn in many formerly competitive states. Missouri, once seen as an electoral bellwether, is now firmly under Republican control. And the fact that most Democrats denounce everything Mr Trump says or does has made these senators’ lives even harder. Yet they are their party’s single best hope of recovery. If the Democrats can defend their seats, and pick up a couple more, probably in Nevada and Arizona, they could take back the Senate. That would give the party control of presidential appointments, including judges, as well as of the legislative agenda. If most of the moderates lose, on the other hand, no amount of grassroots activism in California and New York will dull the blow. “The Republicans could conceivably get to 60 votes,” says Ms McCaskill. “And then, as my grandmother used to say, ‘Katy bar the door’.”

She had just spent an hour on a scuffed stage trying to prevent that, by answering unscripted questions from a small crowd. It was the 52nd such event Ms McCaskill had undertaken in a year—mostly in conservative places where she can at best restrict the size of her defeat. The format suits her. Having held elected office at several levels in Missouri—as a county prosecutor, in the state assembly and as chief auditor—before she entered the US Senate in 2007, she has a good grasp of public policy. She has also retained, unusually for a career politician, a knack of sounding human. She speaks plainly, sounds genuinely interested when she probes the audience for extra details and keeps her crowing to a tolerable minimum.

These skills help Ms McCaskill serve two tactical ends. She can mostly avoid saying nasty things about Mr Trump, who is as popular in Missouri as she is. (“Part of our problem is demonisation,” she says, “It’s like a sugar-high, but it doesn’t get us anywhere.”) They also help her present herself as a pragmatist, eager to work across the aisle—and in the process downplay partisan positions such as her support for abortion. Her likely opponent, Josh Hawley, the state’s 38-year-old attorney-general, is also highly rated. Yet opinion polls suggest Ms McCaskill, though the most endangered Democratic senator, has a decent chance.

Democratic strategists are thrilled. One sees Ms McCaskill’s resilience as proof the Democrats are far more viable in conservative states than is often assumed. If, as expected, the party enjoys a wave of victories in the mid-terms, another predicts its ranks will be swelled by many new moderates, who would peg back the leftish ideologues and further expand the party’s reach. They are probably both half-right, yet much too optimistic.

Even if the Democrats check their drift to the left, the Republicans’ social conservatism will give them a sizeable advantage in the Senate, where small conservative states carry as much weight as big progressive ones. For a Democrat to win in most of the 30 states Mr Trump won requires an outstanding candidate, as Ms McCaskill is, or an awful opponent, like Roy Moore in Alabama, or both. More often, an ordinary pro-life Republican will win.

Nothing in moderation

There is also little evidence of a moderate revival in either party. The newbie Democratic candidates campaigning for the mid-terms are perhaps unusually ideological, the party having deterred pro-lifers and other heretics. And in this, moreover, both parties reflect a shift in the electorate. The essence of political moderation is not to hold mild views, which few voters consistently do, but to be open to persuasion, and only a tiny minority of Americans are. The cohort of swing voters has been falling for decades. Majorities of Republicans and Democrats now have strong negative feelings towards each other. Americans have become hooked on the sugar-rush. It is doing them no good.