U.S. Sen. Dianne Feinstein (D-Calif.) has an 87 percent lifetime liberal rating. The American Conservative Union’s rating for Feinstein, on the other hand, is 9 (out of 100). They call her “part of the coalition of the radical Left.” A former mayor of San Francisco, she is so far left on guns that she supports the confiscation of all (so-called) assault weapons.

“If I could have gotten 51 votes in the Senate for an outright ban … ‘Mr. and Mrs. America, turn ’em all in,’ I would have done it,” she told CBS’ 60 Minutes.

And yet four-term incumbent Feinstein could not get the endorsement of the California Democratic Party in her upcoming re-election bid. Why? She’s not liberal enough.

Earlier this month, U.S. Sen. Liz Warren (D-Mass.) made an unannounced appearance at a national Native American gathering in Washington, D.C. According to Politico, it’s part of a new push to resolve the “Pocahontas” problem that her allies fear will hurt her in a possible 2020 bid. But Republicans are never going to back “Liberal Lizzie.” So why bother?

“Her allies viewed [the speech] as an important step forward for a potential candidate who has faced unexpectedly harsh criticism from … some liberal activists,” Politico reported. Warren’s team sees this issue as “an Achilles heel,” Politico said, not because of Republican mockery but fear of a progressive pushback.

“Liberal Native American activists argue she hasn’t gone far enough in explaining her claimed ties to the culture,” Politico said.

They aren’t alone. Across the country, longtime liberals and die-hard Democrats are moving ever farther to the left in response to the #Resist movement, which equates “being a good Democrat” with “always opposing Trump” and encourages politicians to take ever more extreme positions in opposition.

Remember how close we came to a government shutdown over the DACA immigration issue before Christmas? A group of Democrats voted for a short-term spending measure that didn’t address the DACA issue and instead kept the government open. Feinstein was expected to be one of them.

Instead, in a “surprising reversal” (as the Los Angeles Times called it) Feinstein voted to shut down the government rather than pass a compromise deal to let Congress go home for the holidays.

A few weeks later when the government did shut down, the move was widely seen as a Democratic misstep. Polls showed a majority of Americans were opposed. But the progressive base didn’t care. They loved it. And both Feinstein and Warren gave them what they wanted.

Here in Massachusetts, we’ve already seen Warren go from a candidate who denied supporting socialized medicine in 2012 to one who now adamantly insists we must have a single-payer system. All this hewing to the left, even as Republicans were taking control of the U.S. Senate (2014) and the White House (2016).

This is the Democrats’ dilemma — 2018 is billed as a banner year for Democrats. Political analysts such as Greg Giroux at Bloomberg are predicting Republicans will lose the U.S. House of Representatives beneath an anti-Trump tide and — if it turns into a tsunami — maybe even the Senate.

But if the Democrats become the “Too Liberal For Dianne Feinstein” party; if they embrace Elizabeth Warren extremism as their mainstream (she’s consistently ranked the most liberal U.S. senator), they risk alienating voters they need to win outside blue enclaves like California and Massachusetts.

Consider the gun debate. It’s clear there’s momentum for gun regulations of some kind. President Trump has endorsed ideas from tightening background checks to new age restrictions on gun purchases. Out in America — where a majority continue to support the right to own guns — those measures sound reasonable.

But the activists appearing on CNN are demanding far more. They’re talking about the sort of sweeping bans Feinstein supports. They’re cheering suggestions that all semi-automatic guns — about 80 percent of the entire market — be banned.

That plays well in Wellesley and sounds good in San Francisco. But if Democrats keep pushing, they may find themselves far past Peoria, wondering where their new majority went.

Michael Graham is a regular contributor to the Boston Herald. His daily podcast is available at www.michaelgraham.com.