At issue is whether a demoralized Republican Party should be emphasizing breadth or depth in terms of ideology, messaging and geography.
On one side stand the fiscally and socially conservative purists -- party activists and elected officials who maintain that Republicans need to adhere to strict principles to emerge as a strong opposition party.
Their reaction to the Specter move was summed up by conservative blogger Michelle Malkin: "Don't let the door hit you on the way out."
On the other side stand the beleaguered Republican moderates, the dwindling number of blue-state GOPers and -- less quietly -- key members of party leadership on Capitol Hill.
They maintain that the party can't afford to cut itself off from any blocs of voters, not with a popular Democratic president in office. They warn that the wrong moves now will consign the Republican Party to a prolonged minority status, chasing a shrinking base in ever-smaller portions of the country.
"There is no plausible scenario under which Republicans can grow into a majority while shrinking our ideological confines and continuing to retract into a regional party," Sen. Olympia Snowe, R-Maine -- now one of only three Republican senators from the Northeast -- wrote in the New York Times on Wednesday.
Specter's situation, of course, is unique. He was a moderate senator whose state has grown increasingly Democratic over his three decades in the Senate.
A conservative challenger seemed almost certain to defeat him in next year's Republican primary -- leaving a party switch his only real hope of retaining office.
Specter acknowledged as much in his decision to become a Democrat. But in leaving the GOP, he bemoaned what he described as the party's conservative shift: "Since my election in 1980, as part of the Reagan big tent, the Republican Party has moved far to the right."
Ramifications of Specter's Switch
Specter's defection has huge potential short-term ramifications. With the expected victory of Al Franken in Minnesota, Democrats now stand on the verge of securing 60 Senate votes -- enough to cut off any GOP filibuster.
But some in the party say they're not sorry to see Specter go. Not having a moderate like Specter -- who supports abortion rights and was one of only three GOP members of Congress to vote for the president's stimulus package -- makes it easier for the party to present a united front in opposing Obama.
"I'm happy not to have an Arlen Specter -- a potential traitor at all times -- who can sink Republican messaging because of his ego," said Rick Wilson, a veteran Republican consultant. "He's not a guy who was here for the good of the team."
Obama remains extraordinarily popular in polls as a smaller portion of voters say they consider themselves Republicans. But voters are expressing concern about runaway spending and deficits -- leaving fiscal conservatives convinced that the way to regain voters' trust is through adhering to a strict message of fiscal discipline.
The recent "tea party" protests against the president's tax and spending plans offered a glimpse into the potential political power of government spending as an electoral issue, Wilson said.
Whit Ayres, a Republican pollster, said the party shouldn't try to adjust its fundamental views, when polling suggests that the public is still in line with the GOP's core principles.
"America remains a center-right country, and they perceive Barack Obama as a liberal," Ayres told reporters this week. "They voted for him not because he represented liberal policies, but because he represented 'change.'"
Michael Steele, the chairman of the Republican National Committee, seems to be endorsing the view that the party shouldn't adjust to accommodate more diverse viewpoints. He told NPR Wednesday that the GOP is "still a big-tent party," but he added that that doesn't mean inviting in all comers.
"I'm not going to stand at the door with a little checklist and say, 'Well, you can be a Republican and you can't,'" Steele said. "But understand, it's like, you know, when I come to your house for dinner, all right, and I sit down at your table. What do you think of me when I look at your wife or look at you and go, you know, 'This is a nice meal but I would have preferred chicken. And if you could take this plate off, I think I'd like a different type of china.' It is what you serve."
Steele's perspective appears to reflect the prevailing view in the party's grass roots. Activists' frustration with President Bush on spending and the emphasis on some social issues only grew during the McCain campaign, with the GOP nominee breaking from party orthodoxy on areas including immigration, climate change and campaign-finance reform.
But Republican leaders on Capitol Hill are offering a somewhat different take. While expressing anger at Specter's defection, they are musing publicly about recruiting moderate candidates to run in Democratic-leaning states -- similarly to how Democrats have found conservative party members to run in Republican states.
"This is not a time to purify; it's a time to multiply," Sen. John Barrasso, R-Wyo., said on ABCNews.com's "Top Line" Thursday.
That point of view is colored by the realities of governing: In losing Specter, Senate leaders have lost their best hope of influencing the Obama agenda.
Numbers -- not principles -- are paramount to those who have to try to govern, said Matt Mackowiak, a former GOP Capitol Hill aide.
"If Democrats have 60 votes to put a liberal on the Supreme Court, a lot of activists are going to have to examine how important purity is," Mackowiak said. "A lot of activists are willing to lose the next two or four years -- get rid of some of those guys they think are unreliable, and hope that Democrats overreach so much that the country moves to the right."
That's a major gamble: Republicans risk losing a generation of voters, he said, and are falling behind Democrats in efforts to attract Latinos and political moderates.
But it also puts more pressure on the president and his Democratic colleagues to govern, Wilson said. Moderate Democrats will face pressure from liberal voices to support an agenda that won't be popular in much of the country, he said.
"The left is going to push Obama -- now that he's got a veto-proof majority -- to drive an agenda that a smart president would realize that is a long-term political disaster," Wilson said.
"There will be fights. There will be people who storm off because they don't get their way. But there will people who come into the party because they see us doing things the right way," he said.
ABC News' Teddy Davis contributed to this report.