Conscience forbids us to cross some lines.

Politics, of course, is about compromise. No candidate or lawmaker or president will ever agree with you on everything. Even if he did, real life — actual governing — would force the politician to compromise, to give up some good things for others.

Foreign policy, taxes, corporate welfare, immigration, entitlement reform, national security, civil liberties — all of these issues matter to conservative voters. Some matter more than others.

But the fundamental purpose of government is to protect the vulnerable from violence. There is nobody more vulnerable than a baby. Every year, with our government's approval, the courts' protection, and our own tax dollars, abortionists kill 1 million babies in this country. They kill tens of thousands of babies who have reached the age of viability.

Donald Trump finds that slaughter acceptable. That makes Donald Trump unacceptable.

Trump stands with President Obama and Hillary Clinton in insisting that taxpayers share complicity in this slaughter by continuing to fund Planned Parenthood. His recent claims to a pro-life conversion could not be less convincing.

Not all conservatives care about abortion. Not all Republicans are pro-life. But for those of us who are, a Trump nomination would mean disaster — both major party nominees would favor legal abortion and taxpayer subsidies of abortion.

Trump may argue otherwise, but it's foolish to consider him pro-life.

Many people sincerely change their heart on abortion. Generally, we ought to believe those who profess such a conversion. But Trump is uniquely untrustworthy on this matter.

Trump in 1999 said on Meet the Press, "I'm very pro-choice. I hate the concept of abortion. I hate it. I hate everything it stands for. I cringe when I listen to people debate the subject. But, you still, I just believe in choice."

When host Tim Russert asked him specifically about partial-birth abortion — which Democratic Sen. Daniel Patrick Moynihan had called "close to infanticide," Trump maintained that it should be legal. Joe Biden and Harry Reid both voted the following year to outlaw partial-birth abortion. Trump stood to their left.

In his book the following year, Trump called himself pro-choice, but opposed to late-term abortion.

Trump said this month, "I'm very capable of changing to anything I want to change to." For a Republican primary, he's changed into a pro-lifer. But playing a pro-lifer isn't as easy as he presumes.

Trump has shown himself to know nothing and care nothing about the issue. In 2011, Trump said he agreed with the Supreme Court rulings finding a "right to privacy" in the Constitution, a right invented for the sake of pretending the Constitution protects abortions. Trump had no idea this "right to privacy" was the basis of Roe v. Wade, and so he was totally confused when MSNBC's Savannah Guthrie asked him to square his purported pro-life views with his constitutional views.

When he explains his supposed pro-life views, he mumbles: "It's an issue. I mean it's an issue, and it's a strong issue." Pro-life attorney Kristi Burton Brown wrote of a 2015 interview in which Trump was asked "if he would be against abortion if the life of the mother was not at risk and if rape or incest were not involved (his exceptions), he stated, 'it depends when.'" If, outside of rape or incest, your view on legal abortion is "it depends when," then you are not pro-life.

And no pro-lifer favors subsidizing Planned Parenthood, which in any given year performs 30 to 40 percent of abortions in America. Planned Parenthood exists to abort babies, to fund the re-election of pro-choice Democrats, and to lobby for more government support for abortion. It provides other services, but so do countless other clinics.

Planned Parenthood runs massive lobbying efforts, for more abortion, more abortion subsidies and against the freedom of conscience when it comes to abortion and contraception. Planned Parenthood funds and campaigns for abortion-maximalist candidates such as Hillary Clinton.

Planned Parenthood aborts more than 300,000 American babies per year. Overseas, it does more.

Planned Parenthood is the heart of the abortion industry and the abortion lobby.

Every pro-lifer knows that. Yet when NBC's Chuck Todd asked him if he supports subsidizing Planned Parenthood — as long as the money isn't earmarked for abortions — Trump said, "Yes." He made similar comments last year.

Does anyone trust that Trump would even try to appoint to the court judges who reject the embarrassingly flawed reasoning of Roe and Planned Parenthood v. Casey? Does anyone think Trump's pro-life views would take the form of any action, were he to be elected?

A Republican nominee, even if he or she loses, has an opportunity to articulate directly to voters conservative arguments — arguments the mainstream media rejects and usually does not comprehend. On no issue is this mini-bully-pulpit more important than defense of the unborn.

If Trump is the nominee, the babies will have no advocate. If Trump is the nominee, pro-lifers will have no candidate.

Timothy P. Carney, The Washington Examiner's senior political columnist, can be contacted at tcarney@washingtonexaminer.com. His column appears Tuesday and Thursday nights on washingtonexaminer.com.