Sens. Joe Manchin and Kyrsten Snema. Chip Somodevilla/Getty ImagesSenate Democrats want at least $3.5 trillion to rebuild the economy.They may be stopped by moderates such as Sinema or Manchin.To keep them happy, they may have to lower the cost of their spending plan.Check out more stories from Insider's business page.On Wednesday, Democrats approved a $3.5 trillion spending plan. This laid the groundwork for a major antipoverty push to secure President Joe Biden’s agenda in the coming months.However, the support of all 50 Senate Democrats for the budget resolution gave way quickly to new disagreements within their caucus over the scope and size of the legislative package. Three hours after the vote was over, Sen. Joe Manchin, West Virginia's key swing vote, issued a statement stating that he had "serious concern."He stated that "Given the current economic recovery, it's simply irresponsible for us to continue spending at levels better suited to respond a Great Depression and Great Recession - an economy that is not on the verge to overheating" in the statement.The West Virginia Democrat is not the only one to criticize the spending package's size. He joined Sen. Kyrsten Silena, an Arizona moderate Democrat, in raising concern about the cost. In a statement, she stated that she did not support a bill with a cost of $3.5 trillion.Their hesitation highlights the difficulties that Biden and other Democratic leaders face in keeping rank and file Democrats in lockstep behind the nascent budget plan. They will need to use their thin majorities in both the House and Senate. This may signal that there will be cuts in the price tag needed to keep everyone on board.This is a fact that Senator Majority Leader Chuck Schumer later acknowledged Wednesday. He said, "What we are doing here isn’t easy. We labored for many months and months to get to this point and we don’t have any illusions – maybe the hardest work yet to come." "But we all share a common desire to see it through."Continue the storyThe reconciliation path is a special legislative route that the budget blueprint follows. This track does not require 60 votes as required by the Senate. It requires only a simple majority vote to pass. All 50 Senate Democrats will be required to approve the plan, with a tie-breaking vote by Vice President Kamala Harris. This is likely to overcome the strong GOP opposition.Monday's spending plan would increase Medicare coverage to include vision, hearing, and dental insurance. It would also establish a national paid family- and medical leave program, as well as tuition free community college, a child allocation, and other initiatives to combat climate emergencies. Democrats plan to offer citizenship to some undocumented immigrants who are currently living in the US.Manchin has jumped from one position to the next in an attempt to increase his influence on the legislative process. This is similar to his successful push to reduce the duration and amount of federal jobless assistance in the stimulus law. He stated earlier this year that he would back $4 trillion in infrastructure spending, provided it is paid for. Now, he appears to have reversed that.This proposal is already lower that what progressives had hoped for. The budget resolution was authored by Sen. Bernie Sanders, Vermont, who is the chair of the Budget Committee. Although he initially desired $6 trillion in new spending, he eventually settled for $3.5 trillion. This is despite Congress having approved $6 trillion emergency spending to combat the COVID-19 epidemic.Sanders touts the spending plan, despite the compromise, as the "most consequential piece of legislation we've seen since FDR or the New Deal for working families." Manchin and Sinema could very well be decisive in deciding whether this comes to fruition.Business Insider has the original article.