Ahead of her speech at the March for Life on Friday, Iowa GOP senator Joni Ernst said that Republican frontrunner Donald Trump and other presidential candidates should tell voters where they stand on Roe v. Wade, the 1973 Supreme Court decision that declared a right to abortion-on-demand.

"It is extremely important—extremely important" to have a president who would "appoint judges that understand that Roe v. Wade really wasn't a decision that was based on the Constitution," Ernst told THE WEEKLY STANDARD during an interview in her Senate office Friday morning.

Trump, who used to support a right to partial-birth abortion and third-trimester abortion, now says he's pro-life. He has pledged to sign a ban on late-term abortion and, after vacillating, said he would defund Planned Parenthood. But in December he ducked a question from a voter who asked if he'd seek to appoint judges who would overturn Roe v. Wade. "We're gonna be looking at that very, very carefully, but you need a lot of Supreme Court judges," Trump said.

Asked if Trump should tell voters his position on the issue, Ernst replied that Republican candidates "need to spell that out. They should know where they stand. They're running for the office of president. They should know where they stand on this issue because it is so important."

"I won't formally endorse" a GOP presidential candidate, Ernst added. But she did praise all three of her Senate colleagues running for president—Marco Rubio, Ted Cruz, and Rand Paul—for working with her to defend the right to life. "They are fantastic. They've been very strong on these issues," Ernst said. "I know where they stand."

Ernst said she wouldn't tell Iowa Republicans to oppose any particular candidate, but she did urge them to do their research and "not get caught up in provocative statements."

"I'm not going to tell Iowans how to vote. I think it's important that they make that decision on their own," she said. "So while I'm not going to speak out against candidates, I will encourage our public to dig in, do their research, not get caught up in provocative statements. They really need to know where a person stands on a number of these issues."

"When it comes time and we have our nominee, I'm behind that nominee," Ernst added. "We'll hopefully be able to influence decisions that are made."

Trump's pro-life conversion story and some of his more recent comments have left pro-life leaders with lingering questions about what he really thinks about the issue.

During the first Republican presidential debate, Trump explained that he "evolved" on the issue when he saw an unwanted child grow up to be a "total superstar." Asked if he would have become pro-life if that child had been a loser instead of a superstar, Trump replied: "Probably not, but I've never thought of it." In an interview last year, Trump was unclear about whether he still thinks abortion should generally be legal in the first three months of pregnancy. Trump spokeswoman Hope Hicks has not replied to emails from TWS asking for clarification on these points.

Just last week, Trump floated former senator Scott Brown, who supports a right to abortion, as a possible vice presidential running mate. In 2015, he said he thought his sister Maryanne Trump Barry, a federal appeals court judge who struck down New Jersey's partial-birth abortion ban, would be a "phenomenal" Supreme Court justice. Last Sunday, he pointed to originalist Justice Clarence Thomas as the kind of judge he'd like to appoint.

"We're pro-conversion," Iowa Right to Life executive director Jennifer Bowen told me on Wednesday when asked about Trump's record on the issue. "Some of my closest friends worked in the abortion industry 10 years ago."

"But you can't just say you're pro-life. You've got to go deeper, or it's harder to believe," Bowen explained. "I don't think he's really been vetted well on this issue."