Donald Trump attacked President Barack Obama’s decision not to attend Supreme Court Justice Antonin Scalia’s memorial service Saturday by calling into question the president’s faith.

Trump tweeted Saturday morning that Obama might have attended the service if it had been held in a mosque rather than at the Basilica.

I wonder if President Obama would have attended the funeral of Justice Scalia if it were held in a Mosque? Very sad that he did not go! — Donald J. Trump (@realDonaldTrump) February 20, 2016

Trump competes in the South Carolina Republican primary Saturday where he has been trying to make an appeal to the Republican Party’s most conservative voters. According to a 2015 poll, 43 percent of Republican voters still believe the president is a Muslim despite the fact he consistently refers to his Christian faith in the public arena.

The president has been criticized for his decision not to attend Scalia’s funeral by others, but the White House had defended the decision citing Obama’s extensive security detail as one of the reasons to forgo the service. Obama met with Scalia’s family privately Friday and visited the Supreme Court to pay his respects to Scalia.