Once, it seemed like every Catholic politician in America was running for president.

But no more.

An interesting summary of what happened, and why, from CNS:

Less than a year ago, nearly a dozen Catholics — Republicans and Democrats — had either declared as candidates for president, started an exploratory committee or been widely mentioned as likely candidates.

But with New Mexico Gov. Bill Richardson’s decision Jan. 10 to end his run for the Democratic nomination, Rep. Dennis Kucinich of Ohio remains the most prominent Catholic candidate on the Democratic side.

Democratic Sens. Joseph Biden of Delaware and Christopher Dodd of Connecticut dropped out of the race a week before Richardson did. Early in 2007, Gov. Tom Vilsack of Iowa became the first Catholic to formally end his quest for the Democratic nomination.

Retired Gen. Wesley Clark, once considered a possible candidate for president in 2008, has instead endorsed Sen. Hillary Clinton of New York.

In addition to Clinton, a United Methodist, front-runners for the Democratic nomination include Sen. Barack Obama of Illinois, a member of the United Church of Christ, and former Sen. John Edwards of North Carolina, who was a Baptist growing up but now worships at a Methodist church.

With Kucinich on the so-called “second tier” of Democratic candidates is former Sen. Mike Gravel of Alaska, a Unitarian Universalist.

Among Republican candidates for president, the situation has been even more fluid in the past year. Former Secretary of Health and Human Services and former Wisconsin Gov. Tommy Thompson ended his run for the GOP nomination in August, and later endorsed a fellow Catholic, former New York Mayor Rudy Giuliani.

But another Thompson, former Sen. Fred Thompson of Tennessee, who belongs to the United Church of Christ, has entered the race in the past year, as have former ambassador Alan Keyes, a Catholic, and former Arkansas Gov. Mike Huckabee, a Baptist minister.

Catholic Sen. Sam Brownback of Kansas ended his bid for the Republican nomination in October and endorsed Sen. John McCain of Arizona, who was raised in the Episcopal Church but now attends Baptist services.

Another Catholic, former New York Gov. George Pataki, had been reportedly considering a run for the Republican nomination but never entered the race.

In addition to Giuliani and Keyes, Catholic Republican presidential candidates also include long-shot candidate John Cox of Illinois, but he garnered only 40 votes in the New Hampshire primary.

McCain, Huckabee, Giuliani and former Gov. Mitt Romney of Massachusetts, a member of the Church of Jesus Christ of Latter-day Saints, have been the top GOP vote-getters in early polling.

Jessica Davis, president of Chicago Media Corporation and director of law student development at Southern Illinois University School of Law in Carbondale, Ill., said the main challenge for the Catholic presidential candidates who dropped out was not their religion but “the perception of viability or lack of viability.”

Continue at the link for what exactly “lack of viability” means.