Sixteen years ago, a newly elected Republican president got a huge tax cut through Congress. And he did so with the support of a dozen Democratic senators, most of them politically vulnerable because they came from Republican-leaning states. The mass defection outraged Democratic leaders and grassroots activists, but none of them were really surprised. In the late 1990s and early 2000s, those sorts of Democrats predictably broke ranks on some big issues.

Now, a newly elected Republican president, Donald Trump, is trying to get another huge tax cut through Congress. He and his allies in Congress are having a much tougher time. Even Democrats from the most conservative states want little to do with it, just like they wanted little to do with legislation repealing the Affordable Care Act that Republicans tried to pass over the summer and fall.

Trump and the Republicans could still prevail on the tax cut. They could still return to “Obamacare” repeal, too. But the unwillingness of Senate Democrats to go along with either effort so far is one of the more underappreciated stories of the political year.

That says something important about the skills of Senate Democratic leaders, especially Chuck Schumer of New York. But it also says a lot about Trump, the Republican Party and how their extreme agenda has made it easy for even the most vulnerable Democrats to say no.

A case in point is Democratic Sen. Claire McCaskill of Missouri, a state where voters gave Trump 58 percent of the vote last year. Her net approval rating is barely into positive territory, and she’s likely to draw a strong Republican challenger in 2018. But McCaskill has emerged as a consistent and vocal critic of the tax bill, blasting it as a giveaway to big business and the rich that would hurt most of her constituents.

Republicans have made it easy for the likes of McCaskill to seize the populist high ground on taxes.

McCaskill has said she would support a bipartisan compromise, and as recently as this month, she met with White House officials to discuss the bill. But she has also decried their refusal to alter the bill in any substantial way. Republicans recently added a provision that would repeal the Affordable Care Act’s individual mandate ― a highly unpopular penalty for people who don’t get insurance ― and McCaskill attacked that too, by arguing (correctly) that removing the mandate would mean millions of poor and middle-income people end up without coverage.

McCaskill’s opposition to the bill is already provoking attacks from Missouri Republicans, including Josh Hawley, her possible opponent in the 2018 Senate race. But it’s Hawley who appears to be on the politically weaker side of the debate: The tax cut proposal polls terribly. And it doesn’t take too much imagination to see why the public would have soured on it so quickly ― even relative to the 2001 cuts, which also skewed their benefits toward corporations and the wealthy.

Whatever else former President George W. Bush and his allies did, they didn’t propose to raise taxes on lower- and middle-income Americans so that half of them would end up paying more money, not less, within a decade. They didn’t serve up politically radioactive ideas like cutting stipends that help university janitors pay for their kids to go to college. And they didn’t have the treasury secretary, a former investment banker, and his wife smugly pose for a photo in front of a newly minted sheet of dollar bills. (One pundit said they looked like Bond villains.)

All of this has made it easy for McCaskill to seize the populist high ground, and she’s done just that. “I wanted to support real tax reform,” she said last week, after voting no on the bill in the Senate Finance Committee. “This isn’t it — this is a bad deal for Missouri families. Working people in Missouri deserve better than to get scraps, while corporations and wealthy business owners make out like bandits in a plan that explodes our deficit and compromises our military.”

The debate over health care has played out the same way. Here, the best illustration is probably West Virginia’s Joe Manchin, who is arguably the most conservative Democrat in the Senate caucus. Trump got more than two-thirds of the vote in West Virginia, his second highest percentage in the country, and he even made a personal visit to the state to rally voters behind “Obamacare” repeal. But Manchin wouldn’t go for it. He said he was happy to discuss changes to the program, but only if it protected insurance for those who had it.

And that’s been Manchin’s argument ever since. It’s what he says on television, and it’s what he says directly to constituents, in town hall meetings all over the state. In fact, when Manchin describes helping people get health care as “the greatest gift in the world” ― or accuses Republicans of voting “to throw the most vulnerable in West Virginia out into the cold” ― it’s easy to imagine a Brooklyn twang instead of an Appalachian one, and think that you’re listening to Sen. Bernie Sanders (I-Vt.).

West Virginia has the second-highest number of Medicaid enrollees in the country. Huge cuts were bound to alienate Manchin.

To be clear, the philosophical gap between Manchin and Sanders remains substantial. But the effort to repeal the Affordable Care Act appears to the be the most unpopular major legislative initiative in three decades, and it has brought them together over health care. It’s particularly easy to oppose in West Virginia, which has benefited enormously from the Medicaid expansion. Today, 26 percent of residents are on the program, the second-highest percentage in the country. (Repeal legislation would have both ended federal funding for the Medicaid expansion and cut funding for the rest of the program.)

The surprise isn’t that Manchin took such a firm stand against repeal. The surprise, really, is that Shelley Moore Capito, West Virginia’s Republican senator, didn’t.

It’s tempting to imagine what might have happened if Trump and Republicans hadn’t put forward such extreme ideas on health care and taxes ― if they’d been content to tinker only with the parts of the Affordable Care Act that are truly struggling, or if they’d been willing to give more ground on tax cuts for corporations and the wealthy. But that’s not what the Republican Party is today.

The reality of American politics is that the parties have sorted themselves into more ideologically coherent groups in the last few decades, mostly because of issues related to race, pushing them farther apart from one another. But this polarization of the parties hasn’t been equal on both sides. The available evidence suggests that, at least in Congress, Republicans have become more extreme ― in other words, they are more conservative than the Democrats are liberal.

One way to measure this is by using a widely cited index that a group of political scientists developed and now post at voteview.com. For the current Congress, the mean ideological score for Republicans is 0.49. The corresponding score for Democrats in Congress is negative 0.38 ― which is another way of saying that overall, Republicans are roughly 29 percent more extreme, at least based on their voting patterns.

When Trump became president, it was easy to assume he might pull the party back from the ideological edge, at least on bread-and-butter issues. As a candidate, he kept promising to attack corporations and the rich, while protecting the social welfare programs that middle-class Americans, especially the older white ones that make up his base, value so highly.

But he’s done exactly the opposite since taking office. And unlike Bush, whose approval ratings were comfortably above 50 percent even before the terrorist attacks of Sept. 11, 2001, Trump’s have never hit 50. His approval rating has been hovering in the 30s since May.

There’s every reason to think that Manchin, McCaskill, and the other Democratic senators opposing Republican tax cuts and repeal plans genuinely find the proposals objectionable on substance. But there’s every reason to think at least some of those Democrats would have been a lot more timid if they’d been up against a president whose agenda wasn’t so wildly out of step with the public ― and whose own poll numbers weren’t the weakest in modern history.

CORRECTION: A previous version of this article incorrectly described Trump’s vote share in West Virginia as a little less than two-thirds (it was a little more than two-thirds), and as his highest in the country (it was second-highest, just behind Wyoming).