Joe Biden’s announcement that he will revoke permits for the Keystone XL pipeline if elected is more than just bad energy policy. It’s yet another sign that the former vice president will talk to the center but govern from the left.

Blocking the pipeline has been a cause célèbre for environmentalists for more than a decade. Because it ships crude oil produced from Canadian tar sands in Alberta, they argue that it promotes the exploitation of environmentally dangerous fossil fuels. Environmentalists also argue that this encourages climate change by making another source of greenhouse-gas-causing fuel commercially viable. They succeeded in persuading President Barack Obama to block the project in 2015 by denying necessary federal permits, but President Trump reversed that decision after taking office.

Biden, for his part, has campaigned as a more centrist candidate on energy policy. He’s refused to fully support the Green New Deal, acknowledging the need for oil and gas for the time being and opposing a ban on fracking. But his decision to block an energy project already underway clashes with that message. So, too, does his decision to appoint the hard-line progressive Rep. Alexandria Ocasio-Cortez (D-N.Y.) to co-chair one of his campaign’s committees on climate policy. It is a prime example of Biden acquiescing to the left to court progressive support.

Throughout his career, Biden has moved to the left as Democratic politics changed. This is most clearly seen in his mutating stances on abortion. He began his Senate career as an abortion opponent, going so far as to vote for a constitutional amendment overturning Roe v. Wade. Today, he staunchly backs Roe. He abandoned his 40-plus-year history of supporting the Hyde Amendment, which prevents Medicaid from paying for most abortions, within days of being challenged on his views last summer.

Health care is another issue for which Biden has tilted leftward. During the Bill Clinton administration, Biden resisted progressive entreaties to back universal health-care coverage. Today, he offers a plan that further extends Obamacare and allows people to opt into Medicare at age 60. It’s difficult to imagine him resisting progressive pressure to expand federal control and subsidies further as his term in office progresses.

The list goes on. In the 1970s, he worked with segregationist Southern Democratic senators to oppose mandatory busing, which Sen. Kamala D. Harris (D-Calif.) memorably skewered him for during the Democratic debate last June. Now, he wants to get rid of restrictions on busing. He was against gay marriage in the 1990s, for it in 2012. He advocated increasing the Social Security retirement age; pushed for making sentences for possessing crack much harsher than those for having cocaine; and supported the Iraq War — all of which he has changed his mind on.

In one sense, this is unremarkable. Politicians with long careers often bend with time and, in this, Biden is no exception. What’s remarkable is that the bending always seems to go to the left, and that he takes his new position with a righteous moralism that acts as a perfume to cover the stench of his convenient conversion.

Genuine centrists should be concerned about this well-established pattern. If they back Biden, will they genuinely get someone committed to prudence and national unity? Or will they get someone whose overriding passion is to be acclaimed by his party no matter he has to say to win the applause?

Trump would do well to focus on this when he starts to define Biden. Biden’s frequent gaffes and misstatements are worrying, but Trump’s own verbal inaccuracies and serial misstatements make him a less than credible critic. Going after Biden as a crazy liberal would also likely backfire. The presumptive Democratic nominee has long practiced the art of trending left but sounding reasonable, keeping just enough room between his latest deepest conviction and those of the left to maintain plausible deniability. These are only two of the reasons why Biden is a difficult foe for Trump, provided he maintains a minimal veneer of competence.

Trump should instead attack Biden for what he is: a weak and vain man who adopts the views of those whose approval he craves. It’s a truer charge than the alternatives and has the added advantage that it will wound Biden’s pride. Like many pliant people, he is convinced he is strong and decisive and is likely to lash out in fury at any insinuation that he is not. An angry Biden makes mistakes, and Trump needs Biden to make mistakes to convince the few wavering voters that they should prefer him despite their misgivings.

Woody Allen’s film “Zelig” starred a man with such a strong a desire to be liked that he would mimic and imitate the views of people around him with uncanny success. Biden’s Zelig-like ideological transformations have landed him within sight of the White House. Don’t be surprised if he falls short once the reality is unmasked.

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