After aggressively whipping votes, the office of House minority whip Steny Hoyer now believes that not a single Democrat will vote for John Boehner’s debt ceiling plan, delivering a blow to GOP hopes for getting it through the House, a top Democratic aide tells me.

“Hoyer has been whipping against the Boehner bill very hard,” says the aide, who is close to Hoyer’s whipping operation. “No Democrats will vote for it.”

If this bears out — and it’s important to emphasize that nothing is ever certain in the alternate universe otherwise known as the United States Congress — it could be a very big deal. It means that House Republicans need all Republican votes to pass the Boehner plan, which would hike the debt ceiling in stages, and significantly reduces the margin of error House GOP vote counters have at their disposal.

Each Dem vote for the plan means GOP leaders need one less House conservative to support it, and if Dems unanimously vote against the proposal, House Republicans can only afford to lose roughly two dozen GOPers in order to pass it. The latest public whip count has it that some 20 House GOPers are either firm No votes or are leaning in the direction of No, though the private count could be somewhat different, and it’s also important to remember that things could change once GOP leaders release the new version of their proposal in response to the CBO score finding less-than-desired savings.

If Dems are indeed holding the line against the Boehner plan, it means a couple of things. First, the CBO’s verdict — it found that Harry Reid’s proposal reduces the deficit significantly more than Boehner’s does — is turning the tide against Boehner’s proposal, and leading to a consensus among Dems that they can force the issue that the Reid plan is the true compromise measure.

Second, it means Dems are determined to maintain total unity against the Boehner plan, because they recognize that defeating it is absolutely key to their endgame strategy. If Boehner’s proposal goes down, Dems can point to Boehner’s inability to pass the plan in order to exert more pressure on Republicans to back Harry Reid’s plan, or a variation of it, as the last remaining way to avert default and economic catastrophe.

Again, anything can change, and Democrats may be leaking their whip count as a bluff. But if this bears out, it’s very good news for Dems.

UPDATE: At a preser today, Harry Reid also vowed that no Senate Dems would vote for the Boehner plan, claiming:

Every Democratic senator will vote against it. Nobody believes the Boehner plan is anything more than a big wet kiss for the right wing

Dem unity is important for another reason: If Boehner’s plan does pass, but with no Dem votes, it will be easier for Dems to portray it as a sop to the Tea Party that doesn’t have any bipartisan support.