A massive plan to expand social programs like Medicare, tackle climate change and fund pre-K and community college cleared its first hurdle in the Senate early on Wednesday, kicking off an intricate process to pass a $3.5 trillion package that is a signature part of President Joe Biden’s agenda.

The budget resolution passed the Senate at just before 4 AM ET on a party line vote, 50-49.

Less than 24 hours earlier, the Senate passed a $1.2 trillion infrastructure package that passed on Tuesday with 19 Republicans joining with all 50 Democrats and independents. That was a rare moment of bipartisanship for such a massive $1.2 trillion package, but that sentiment quickly gave way to Republicans’ united opposition to the larger $3.5 trillion budget resolution.

The House will return to D.C. the week of Aug. 23, which will make for a political high wire act as Democrats try to coalesce around both spending plans. If they were to ultimately pass, it would be a historic expansion of the social safety net, perhaps to an extent not seen since the 1960s.

The budget resolution is basically a blueprint. Senate committees now will draw up details of the spending plan for a final package, which could then pass by simple majority. That process is expected to take until mid-September.

Democrats are proposing to fund the plan in part via a boost in the corporate tax rate — something opposed by the business lobby, including Hollywood studios — and by increases on taxes for high income earners.