A filibuster-proof 64 senators on Friday sent a letter to President Obama, asking him to engage in serious discussions on a broad plan for long-term deficit reduction. Encouraging. But another letter went out this weekend, from the Tea Party Patriots to its supporters, and it highlights an ironic but distinct possibility: Even if centrists unite around a real plan, will the Tea Party or other conservatives kill the sort of deficit-cutting compromise — and there must be compromise for a long-term budget plan to pass, let alone for it to be durable — that those 64 senators envision?

The senators' letter, carrying the signatures of an equal number of Democrats and Republicans, encourages Obama to consider a plan that would include "discretionary spending cuts, entitlement changes and tax reform." The language is pretty cautious — entitlement changes, not cuts; tax reform, not increases — but the letter cites the president's fiscal commission, which was far less euphemistic in its call for more taxes and fewer entitlement benefits. So, despite some pundits' head-scratching, it's another positive signal that there is appetite in the Senate for a long-term budget plan that contains pain for each side in the debate. The White House, meanwhile, has welcomed the letter, apparently eager to see a bipartisan alternative to the rhetoric in the House of Representatives.

So what about the conservative House? The Tea Party Patriots are among the groups in the GOP base that pose a threat to compromise in that chamber. As if to remind us, its leaders over the weekend began rallying support for a march on Washington later this month, expressing dismay that the Republican House hasn't defunded health-care reform or cut enough from the budget yet.

"Your sacrifices," the solicitation reads, "are the reason that the GOP holds the gavel today. You put them in office, and yet they aren't listening to you, and they're not listening to those in their caucus that would dare to tell the truth." Other Tea Party groups republished this letter on their Web sites.

Merely attempting a total repeal of Obamacare, threatening not to raise the debt ceiling and engaging in shutdown-watch brinkmanship on the current year's budget, it seems, isn't satisfying this section of the base. "We also realize that all of us hoped that changing the make up of one chamber of Congress in November would prevent us from having to make emergency trips to DC. Now we realize that may have just been wishful thinking." Not a good sign for how these Tea Partyers will behave in a debate over long-term budget reform, given that the stakes then will be incalculably higher than they are now, in the current wrangling over funding just for the rest of this fiscal year.

Sure, the Tea Party Patriots is just one interest group; it's not even the whole Tea Party. Yet its Web site hosts strategizing forums on how to "dominate" GOP primaries in the 2012 election, its contributors emboldened by 2010's Tea Party success in determining Republican nominees across the country. Many in the GOP House caucus, cautious of their right flank, are bound to listen, and the prospect for a real, long-term budget deal any time soon depends heavily on how much they do.