Even before this afternoon’s rally, where Florida Governor Charlie Crist is all but certain to announce that he will exit the Republican party and seat his state’s open U.S. Senate seat as an independent, the candidate’s official website featured a telling frontpage headline: “Register to Vote/Switch Parties.”

Crist, though barely a moderate in any traditional sense, has always tried to appeal across lines of partisanship and, to some extent, ideology.

That used to be acceptable within the Republican Party of Ronald Reagan, who went out of his way to champion a “big tent” vision of the Grand Old Party, and even of George W. Bush, who backed the reelection of moderate, labor-friendly Republicans such as Pennsylvania Senator Arlen Specter in primaries with conservative challengers.

But the big tent has been folded, as Crist learned when the popular governor was scorned by rightwingers for seeking stimulus federal funding for his state and even — horrors — embracing a Democratic president who had come to deliver those funds.

So Crist is leaving the Grand Old Party, declaring that: “Our political system is broken.”

To the cheers of supporters, he declared that he would run as an independent in November’s Senate race because: “I believe in democracy and the right to choose.”

That’s a good line. But this is not about believing in democracy. This is about the fact that Crist cannot win a Republican primary because the party base thinks he is too liberal, too moderate or, at the least, too bipartisan to be their candidate.

Crist is no liberal in the tradition of Republicans such as former New York Senator Jacob Javits, former New Jersey Senator Clifford Case or former Connecticut Senator Lowell Weicker — let alone former New York Mayors Fiorello La Guardia or John Lindsay.

Crist is not even a moderate in the tradition, say, of a former Massachusetts Governor William Weld or one of the two remaining sort-of moderates in the Republican Senate Caucus, Maine outliers Susan Collins and Olympia Snowe.

Crist is best understood as a ideological inheritor of Senate Majority Leader Bob Dole, R-Kansas, or, perhaps, former President George Herbert Walker Bush. For instance, Crist supports capital punishment and gun rights and opposes same-sex marriage and, though he opposes overturning Roe-v-Wade, he has appointed anti-choice jurists to the Florida Supreme Court.

That’s hardly the profile of a social moderate in any realistic sense.

But it has been the profile of a winner. Crist has been repeatedly elected in Florida, to the legislature and as the state’s education commissioner, attorner general and governor. In good years for Republicans and bad, he has been the party’s standard-bearer and frequently its unexpected winner.

What makes the Floridian appealing is not so much his moderation as his relative sanity.

Crist has refused to go off the ideological deep end that has claimed so many formerly mainstream conservative Republicans — making the party’s U.S. House and Senate caucuses over as “party of no” preserves of reaction rather than functional engagement with the governing process.

Along with California Governor Arnold Schwarzenegger, Crist acknowledges that climate change is a threat and proposes to address it.

He’s for conservation and critical of offshore drilling.

He’s mildly sympathetic to public education.

If anything has distinguished Crist, it is his determination to reach out to African-American and Latino voters, as well as other ethnic and racial minorities.

Crist is proud of the fact that he was the first Republican governor to accept an invitation to a convention of the state’s National Association for the Advancement of Colored People (NAACP). And in response to complaints that African-American votes were being undercounted by computerized voting machines, he broke with fellow Republicans to back legislation requiring paper records of all ballots cast during an election.

After the GOP suffered heavy losses in 2008, Crist went out of his way to suggest that the party needed to broaden its reach.

“This party can no longer hope to reach Hispanics, African-Americans and other minority groups — we need to just do it,” he told the Republican Governors Association meeting in Miami that November. “Embracing cultures and lifestyles will make us a better party and better leaders. This desire for inclusiveness is near and dear to my heart…. Last week, the American people made a choice and this week, if we choose to call ourselves leaders, if we truly endeavor to serve with a servant’s heart for the people who count on us, then we too must work together, listen to one another and learn from the leaders who made the kind of history the American people deserve.”

That message stood him in stark contrast to the likes of then-Alaska Governor Sarah Palin.

Crist was dismissed as a “Republican-in-Name-Only” (RINO) and talk about mounting a “true conservative” primary challenge to the popular governor’s Senate run mounted. Former Republican legislative leader Marco Rubio fit the bill. Palin, while withholding a formal endorsement, declared: “I love Marco Rubio. Conservatives flocked to his candidacy and the signal was sent that it was “RINO hunting time.”

The stronger signal, of course, is that the Grand Old Party is no longer sending out invitations. Yes, if regions where the party is no longer viable at the presidential level — especially New England, to a lesser extent Illinois and, perhaps, California — a few moderate conservatives are still tolerated (especially if they can pay for their own campaigns.)

But Crist’s exit is another signal that the Republicans are becoming a closed camp where there is little room for the moderates or even for those who might dabble in the mainstream. And there is certainly no space for Ronald Reagan’s big tent.

There may be a point at which the Republicans scramble back from the right edge of our electoral sprectrum. But that is not the issue this year.

This year, the question shifts to whether relatively rational players such as Crist can find a place in our politics.

The Republicans and Democrats are both spinning the line that Crist is merely ambitious. Well, duh! Of course, he’s ambitious — just like the Democrats and Republicans who are running.

That’s not the point. It never was.

The point is that our politics, to be functional, must entertain a variety of views. And our parties have traditionally allowed for significant ideological variations — usually, although not always, along regional lines.

If the parties are no longer flexible, the question is whether the broader electoral system can open up enough to allow for a rich and nuanced debate that might entertain proposals that are both reasonable and radical, both rational and responsive to the popular will.

Because of Crist’s move, Florida will be the testing ground.

If Crist is defeated by Rubio, the right will claim its party.

If Crist and Rubio lose to Democrat Kendrick Meek — and especially if Rubio finishes third — there will be plenty of fingerpointing followed by, perhaps, a rethink among responsible Republicans of whether it is wise for any political party to place ideological purity above electoral viability.

If Crist can carve out a plurality of the vote in a November race with Republican Rubio and Democratic Meek, the process will be opened — a bit. And 2010 might yet be a year of independent breakthroughs, with veteran Republicans officials who have fled the party shaking up elections not just in Florida but in Rhode Island, Michigan and other states. Throw in a few Green, Libertarian and Vermont Progressive Party victories and we’re talking about change you can believe in.

One does not need to be on the left or the right to recognize that there are benefits to opening up our system. And one does not need to like or agree with Charlie Crist to recognize that his bold move might help to do that.