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The video: Former Pennsylvania Sen. Rick Santorum (R) made it official Monday, formally announcing his candidacy for the Republican presidential nomination. In an appearance on Good Morning America that preceded a speech in Somerset, Penn., Santorum said his campaign has a "lot of momentum" in early primary states, and that he's "in it to win." (Watch the video below.) A devout Roman Catholic who served in the House of Representatives from 1991 to 1995, and in the Senate from 1995 to 2007, Santorum is best known for his strongly-worded, and sometimes controversial, stances against abortion and gay marriage.

The reaction: Santorum's "boldness" has made him a divisive politician, says Ari Pinkus in The Christian Science Monitor. The question now, reinforced by his 17-point loss in his 2006 Senate re-election bid, is whether Santorum is mainstream enough to win a presidential election. He "barely registers" in national polls, says Maeve Reston in the Los Angeles Times, and if he can't convince conservative GOP voters — his ideological allies — "that he has a chance of winning," even they may not back him. Don't underestimate Santorum's appeal, says Hugh Hewitt at his blog. He has an "endless capacity for the hard work of campaigning," which is just "the sort of virtue that succeeds in Iowa and New Hampshire." Watch Santorum on Good Morning America: