One of the reasons that so many massive spending programs were able to be quickly initiated during the New Deal, and the reason that the United States was able to scale up its wartime industries so quickly for World War Two, was due to a special fiscal entity that no longer exists -- the Reconstruction Finance Corporation, or RFC. It was actually first promulgated in the Hoover Administration to help pay for public works, but it was FDR who instantly seized on the opportunity it presented, and he appointed an accomplished Texan banker and Democratic Party stalwart, Jesse H. Jones, to lead the RFC with the intention of dramatically increasing spending.
The RFC is the secret sauce of the New Deal. You've likely read previously that 1932 was a massive Democratic sweep, so that FDR had a Congress that could quickly pass a large number of reform bills, but you also likely have heard that during this same period, many Southern Democrats were famously stingy and were often an obstacle. The RFC was the mechanism that enabled FDR to spend so much money on so many different enormous projects simultaneously, because it bypassed Congressional appropriations completely. Most of the money did not require any spending through Congress!
Instead, the RFC was given its money directly by the Federal Reserve, and the RFC would provide funding to the WPA, the PWA, the CCC, the TVA, etc. The RFC under Jesse Jones kept a careful watch over its ledger, and in turn the Fed kept an eye on the RFC to ensure things didn't get out of control. And one step further, if you look at the Public Works Administration, under the control of the famous Chicago good-government reformer Harold Ickes, he was very careful about the money he was given for his projects as well. In fact, one of the reasons the WPA was created under Harry Hopkins was because Ickes wasn't getting the project money flowing quickly enough so a second money faucet was created.
The RFC bailed out community banks across the country and typically made a return on its investment. It helped pay for farm relief and recapitalize factories. It paid for infrastructure, urban renewal, public housing, private housing developments, wastewater and drinking water, rural electrification, and when we began building the Arsenal of Democracy for World War Two, it made the payments to the privately-owned factories to retool them for wartime production, among many other enormous wartime projects.
It was figuratively a money hose. The title of Jesse Jones' autobiography is Fifty Billion Dollars for a reason -- they knew it was a staggering amount of money they were putting into circulation. You may be wondering why this didn't lead to serious inflation. Well, it did. It didn't spiral into hyperinflation due to wartime price controls, wartime rationing which deliberately reigned in consumption, and war bond drives. War bonds were helpful in locking up the wages of all of these millions of newly-unionized wartime industry workers and slowing down the velocity of money. But inflation was sufficiently high that (1) Harry Truman dealt with a lot of wage-related strikes, economic instability, and low popularity ratings during the postwar, (2) the government was able to pay off all of its war debt, especially its war bonds, by basically allowing the real value of the debt held by its creditors (including the war bonds of the American people) to diminish via inflation. Whoops!
Naturally periods of inflation are good for debtors and bad for creditors. Republicans pushed through two changes that killed the RFC and its partnership with the Fed, and led to the undoing of this fiscal policy that had built so much of the American 20th century infrastructure. Firstly, they passed a mandate for the Federal Reserve that required it to prioritize combating inflation. Implicit in that mandate is you can't be spending money if you think it'll tick up inflation. Secondly, the Republicans retook the US House and Senate for a single term from 1953-1955. With that short window of opportunity, and against the protests of Senator John F. Kennedy and others, they passed a law to eliminate the RFC.
It isn't difficult to see why other major spending initiatives, such as Johnson's War On Poverty, and the various full employment initiatives and infrastructure spending plans, typically promoted as another New Deal or a "Marshall Plan For America's Cities," have failed: these plans all depended on Congressional appropriations, whereas the RFC enabled Roosevelt to go around Congress -- oftentimes to the delight of the members of Congress who wanted projects in their districts, but didn't want to raise taxes or deficits.