approved both the health care bill and the budget measure on March 21, 2010, a Sunday. The Senate followed with its own approval of the budget bill the next Thursday, with Mr. Biden, then the vice president, presiding over the Senate to handle procedural fights and be on hand for the conclusion of a tortuous process.

“The bill as amended is passed,” said Mr. Biden, employing a bit of low-key Senate vernacular at the climatic moment.

Circumstances today are vastly different, and the budget bill taking shape is much more sweeping. Democratic leaders have linked the two measures as “hard” infrastructure and “soft” infrastructure, but Democratic moderates — and many Republicans — see the bills as distinct ventures.

The drafting of the social policy bill under the protection of budget rules is likely to touch off numerous Democratic clashes over cost and scope. Complicating the effort is the fact that Democrats have much thinner majorities now and will not be able to afford defections. In 2010, three Democratic senators abandoned the budget bill in the end and almost three dozen House Democrats bolted. If Senate Democrats today lose a single vote, the effort will die.

Senators Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona, both Democrats, have both sounded the alarm about the cost of the Democratic budget plan though they joined the rest of their Democratic colleagues early Wednesday to begin drafting the legislation.

In the House, Ms. Pelosi also has far less room to maneuver than she did in 2010. She already faces a push-and-pull between progressives demanding maximum spending and more aggressive policy from the reconciliation bill and centrists anxious about the spending level who would prefer that Democrats take the bird in the hand and pass the infrastructure bill as quickly as possible. House moderates indicated on Wednesday that, at least for now, Ms. Pelosi did not have the votes to pass the Senate’s budget blueprint.

The need to hold on to every Democratic vote ultimately gives all lawmakers leverage to make demands as they did in 2010 when some anti-abortion Democrats nearly derailed the health care bill before reaching an accommodation with Ms. Pelosi and ultimately backing it. No doubt today’s bill will be in dire straits and then rescued repeatedly.

As for Republicans, they will do all they can to frustrate the Democratic effort and promote Democratic discord.

Enacting policy through reconciliation has other pitfalls as well; the Senate’s complicated budget rules enable supporters to go only so far. The Affordable Care Act suffered for years from deficiencies that could have been avoided had it been enacted in the traditional way and smoothed out as it made its way through the process.

But Democrats see no alternative to the path they are headed down and are hoping to repeat the success of 2010.

“I am very pleased to report that the two-track strategy is right on track,” Senator Chuck Schumer, Democrat of New York and the majority leader, said Wednesday as he celebrated the twin legislative victories on infrastructure and the budget.

Keeping it there will be very difficult.


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