Picking Supreme Court justices was already a sticky wicket for Donald Trump, and Justice Antonin Scalia's death this weekend is making it even harder for the billionaire GOP presidential front-runner.

Trump's opponents, especially Sen. Ted Cruz, argue that he can't be trusted to pick a conservative justice. Cruz said Sunday on NBC's "Meet the Press" that Trump would pick a liberal justice just as would Democrats Hillary Clinton or Bernie Sanders.

"Just as Ronald Reagan was to the presidency, so was Scalia was to the Supreme Court," Cruz said. "He had that big an impact, and I think his passing yesterday really underscores the stakes of this election."

Trump previously indicated he might nominate to the court someone like his sister, a federal judge who once helped strike down a partial-birth abortion ban in New Jersey. That statement, combined with Trump's past support for some Democrats, has Republicans questioning whether he would pick conservatives for the court, which will likely see more vacancies in the next few years.

Appearing Sunday on "Meet the Press," Trump admitted that ensuring a Supreme Court justice will rule conservatively is difficult.

"You never know what happens," Trump told host Chuck Todd.

But Trump slammed Cruz for supporting the nomination of Chief Justice John Roberts, a tactic he's been trying over the last few weeks. Trump has faulted Cruz for Roberts' decision upholding President Obama's controversial healthcare law. Trump notes Cruz supported Roberts' nomination to the court in the first place.

The real estate developer doubled down on those attacks Sunday.

"You look at what a guy like Ted Cruz, who pushed for Justice Roberts," Trump said. "And Roberts let us down by approving Obamacare twice, and that was largely Cruz's fault, that was a Ted Cruz mistake because he pushed him very hard."

Cruz defended his support of Roberts, but said Roberts wouldn't have been his first pick for the court, saying he would have instead nominated federal judge Mike Luttig, his former boss.

Trump also threw out the name of someone he might appoint if elected president: federal judge Diane Sykes. But he didn't give details when pressed by Todd to explain whether he would apply a litmus test to a potential nominee to ensure they're a conservative.

"Certainly we have some great people out there," Trump said. "I would want the person to be tailored to be just like Justice Scalia."