PHOENIX A surging Rick Santorum is making increasingly harsh remarks about President Barack Obama, questioning not just the president's competence but his motives and even his Christian values.

Mitt Romney also is sharpening his anti-Obama rhetoric. He said Tuesday the president governs with "a secular agenda" that hurts religious freedom. In general, however, the former Massachusetts governor has not seriously challenged Obama's motives, often saying the president is decent but inept.

But Santorum and Newt Gingrich have heightened their claims that Obama's intentions are not always benign, ahead of tonight's televised GOP presidential debate and next week's primaries in Michigan and Arizona.

Santorum says Obama cares only about power, not the "interests of people." He says "Obamacare," the health-care overhaul Obama enacted, includes a "hidden message" about the president's disregard for impaired fetuses, which might be aborted.

Santorum even seemed to compare Obama to Adolf Hitler, although he denies trying to do so.

Santorum's remarks have gotten only scattered attention because he weaves them into long, sometimes rambling speeches. Romney's team is monitoring Santorum's comments, privately suggesting they could hurt him in a general election.

But it's difficult for Romney to openly criticize Santorum on these points because Romney already has trouble appealing to the party's socially conservative base.

Gingrich, campaigning Monday in Oklahoma, called Obama "the most dangerous president in modern American history." Gingrich said the administration's "willful dishonesty" about alleged terrorists' motives threatens the country.

Gingrich has long been known for over-the-top rhetoric, and Santorum's rapid rise in the polls has drawn much of the campaign's focus away from the former House speaker.

Some of Santorum's remarks echo attacks on Obama during the 2008 presidential race, when critics portrayed him as a mysterious politician with hidden motives and questionable allegiance to the United States.