Facing last-minute resistance from members of his own party, House Budget Chairman Paul Ryan will dive headfirst into turbulent water in releasing his fiscal 2013 budget today.

The Wisconsin Republican plans to mark up the resolution Wednesday, and he will need near-unified GOP support to pass it out of committee. But it remains unclear whether he has the votes.

With hours to go before the budget’s rollout, hard-line conservatives spent Monday mounting an eleventh-hour charge against the proposed $1.028 trillion spending level and the amount of time that the plan would take to balance the budget, according to several GOP aides familiar with the situation.

“I’m still not sure how this gets out of committee at [$1.028 trillion] with no balance for a quarter-century. I really don’t,” one aide said to a conservative Member.

The situation has the potential to end up much like the bungled transportation legislation, with leadership dropping a bill before rounding up the votes to pass it. In the Budget Committee, it would take only three disenfranchised Republicans to sink the bill if, as expected, all Democrats vote against it.

Tea-party-affiliated groups such as FreedomWorks sent a letter to House Republicans last week indicating that they would not support a budget that takes more than a decade to balance.

But Ryan has rounded up some support.

Despite pushing for weeks to keep the budget’s topline number in line with July’s Budget Control Act, some Republican appropriators, three of whom sit on the Budget Committee, seem to have settled at the $1.028 trillion spending cap. Rep. Tom Cole, one of those three Members, said he was willing to accept a spending level lower than the law’s $1.047 trillion in exchange for a budget that deals with sequestration through cuts to entitlements rather than discretionary spending.

“We’ve coalesced at what we think is the reasonable figure,” the Oklahoma Republican said. “We ended up at a realistic point. Everybody had to give up things that they didn’t want to give up.”

But conservative Members were still vexed Monday by the plan to alter the sequestration process. The budget might shift tens of billions in would-be cuts to discretionary spending to the mandatory side of the ledger. Some members of the conservative Republican Study Committee argue that mandatory spending cuts should come in addition to the sequester’s $97 billion in discretionary spending cuts for fiscal 2013.

Dropping a budget that RSC members openly reject would be essentially daring them to kill it in committee, a move that would open them to attacks from Democrats, whom the GOP has long chided for not passing a budget.

Democrats are already eager to score messaging victories off of Ryan’s budget, with the Democratic Congressional Campaign Committee charging in some vulnerable GOP districts that Republicans want to “end Medicare.”

On a conference call with reporters Monday, House Democrats echoed the line, with Caucus Vice Chairman Xavier Becerra (Calif.) saying Ryan’s budget “has called for the end of Medicare as we know it.”

Ryan looks poised again to try to reform Medicare by installing a “premium support” system of vouchers from the government for beneficiaries to use to pay private insurers.

Ryan and Sen. Ron Wyden (D-Ore.) came out with a variation of the measure back in December, and Ryan could again turn to Wyden as his chip for bipartisan support.

But Rep. Jan Schakowsky said on the Democratic conference call that there is “no difference in principle” between the original Ryan plan and the Ryan-Wyden plan.

“Both are, in fact, equally bad or only marginally different but still would end Medicare as we know it,” the Illinois Democrat said.

Still, the first wave of Democratic action against whatever Ryan puts forward today will focus on the fact that it deviates from the BCA.

On Monday afternoon, Senate Budget Chairman Kent Conrad (D-N.D.) and Senate Appropriations Chairman Daniel Inouye (D-Hawaii) sent a letter to Speaker John Boehner (R-Ohio) and House Majority Leader Eric Cantor (R-Va.) urging them to keep their agreement on the spending levels. The two top Democrats said that failing to uphold the Budget Control Act deal “would risk a government shutdown” by delaying action on this year’s spending bills.

“We believe that ignoring the BCA represents a breach of faith that will make it more difficult to negotiate future agreements,” Conrad and Inouye wrote. “Rather than trying to tear down the BCA, we should be holding it up as an example of what can be accomplished if we are willing to set aside our differences and work hard to find bipartisan solutions to our nation’s challenges.”

Conrad will hold a briefing today just minutes before Ryan is scheduled to release his budget to file the deeming resolution setting the spending levels for the 2013 fiscal year, as required by the Budget Control Act. The move also serves as a signal to the House that tinkering with the agreement will jeopardize final passage in the Senate.

Privately, however, Democrats are banking that if Republicans move to reduce spending levels, they will make the political situation more difficult for themselves if they are forced to retreat to the original agreement that received significant bipartisan support in August.

“The fact that they’re even having to relitigate it, and to do it so publicly, I think is problematic for them,” a Democratic aide said.