Baffled by Donald Trump’s persistent lead in the polls, establishment Republicans are scrambling to find arguments with which to stop him. So far they have hit him not from the right but from the left, calling him a heartless nativist, a sexist boor, and a reckless hawk. This has only served to drive him deeper into the arms of conservatives.

Meanwhile, to confuse matters even more, Trump attacks establishment favorites such as Jeb Bush not from the right but from the left, accusing him, for example, of insufficient support for big-government funding of “women’s health issues.” Interpreting that phrase as a euphemism for funding Planned Parenthood-like activities, Bush had said in an interview that he didn’t favor it. Ever since then, Trump has cast it as a campaign-ending gaffe, a criticism one would expect from an MSNBC liberal, not from a conservative Republican. “What he said was unbelievable,” said Trump. “Essentially he said he was not going to fund it. I think that will go down as Jeb Bush’s 47% — the 47% to Romney that probably cost him the election.”

Haplessly, Bush hasn’t bothered to engage Trump on this matter. Bush could have used it to question Trump’s conservative credentials, noting that a Trump administration would continue to support Planned Parenthood. “We have to look at the positive things as well for Planned Parenthood,” Trump has said.

Such statements would be discrediting for any other candidate presenting himself as a conservative Republican. But Trump gets away with it, thanks in large part to the silence of his opponents. At this point, Trump isn’t even bothering to conceal his heterodox views, breezily telling Chuck Todd last Sunday that he is “fine” with affirmative action and misrepresenting Ronald Reagan’s support for abortion in cases of rape and incest (Reagan didn’t). Trump is apparently confident that none of his establishment rivals will challenge him on these points.

In a way, Trump has hoist them by their own petard. For years, they touted the importance of bringing idiosyncratic Republicans into the Big Tent. Now they have one on center stage and they don’t know what to do with him. Having dismissed the value of adherence to the party’s platform, they find it difficult to call Trump out for departures from it. They find it easier to level politically correct criticism at him, which ends up inoculating him against serious conservative scrutiny.

Trump’s immigration plan this week draws them back into this self-defeating criticism. While his heterodox opinions go unexamined, his straightforward position on illegal immigration gets more and more attention, making him seem like the conservative outlier in the race. As long as this coverage continues, Trump can take as many liberal positions on other matters as he likes and pay no price for it.

Primary voters will not reject Trump for his alleged nativism, but they might reject him for his statism, both past and present. Of course, that would require his opponents to highlight it, which they appear loath to do, as many of them share it. When Trump boasts that “Democrats will love” him for the way he would “take care” of women and the poor, they feel less concern than envy. They don’t question the big-government premises of such declarations; they too would like to see themselves as a “conservative with a heart,” which is Trump’s description of himself.

Under George W. Bush, conservatives learned the meaning of this compassionate conservatism: a federal government just as bloated as the one under the Democrats. One would think Trump’s opponents, just out of political opportunism, might warn conservatives about the bitter surprises and disappointments in store for them under a Trump administration. But even with an opening as large as the one presented by Trump’s past support for a special wealth tax, they don’t take it. They would prefer to carp about his “tone” and other trifles.

Even though it cuts against their moderate instincts, they will have to fake up an interest in Trump’s deviations from conservatism and intensely engage them to upend his campaign. It is his positions, not his personality, that makes him vulnerable. He has thrown out just enough red meat to keep conservatives listening but not enough to seal their support. But if the race remains one of Trump versus pale establishment moderates, primary voters may just throw up their hands and cynically conclude that if the party is going to lose it might as well lose with the boldest in the bunch.