The U.S. Senate may have passed President Joe Biden's $1.2 trillion infrastructure bill with bipartisan support, but internal divisions among Democrats continue to jeopardize the bill's chances of reaching the president's desk.

Shortly after the bill passed Tuesday, with 19 Republican senators voting for the infrastructure deal, a group of progressive Democrats in the House sent a letter to Democratic leadership threatening to withhold support for the bill in the House until the Senate passes a larger $3.5 trillion budget bill funding the rest of the left's priorities.

The Congressional Progressive Caucus letter, addressed to House Speaker Nancy Pelosi (D-Calif.) and Senate Majority Leader Chuck Schumer (D-N.Y.), stated that a survey of the 96 lawmakers in the caucus found that a majority of them will not vote for the $1 trillion infrastructure deal until the Senate passes the $3.5 trillion bill.

"These results affirm the urgency of ensuring that the Senate's desire to pass a narrower bipartisan infrastructure agreement does not come at the expense of the full scope investments our communities need, want, and deserve," Congressional Progressive Caucus Chairwoman Rep. Pramila Jayapal (D-Wash.) said in a statement.

She added: "Our Caucus is clear: the bipartisan bill will only be passed if a package of social, human, and climate infrastructure — reflecting long-standing Democratic priorities — is passed simultaneously through budget reconciliation. We know that Congressional Democrats are committed to delivering immediate and transformational improvements for the lives of the American people, and will hold firm to meet that promise."

Senate Democrats chose a two-track strategy to advance massive government spending bills they call "infrastructure" packages. The $1.2 trillion bill that passed with bipartisan support on Tuesday includes funding for "hard infrastructure" — roads, bridges, and other projects traditionally associated with infrastructure spending — along with various other spending critics say is unrelated to infrastructure.

The second $3.5 trillion package is a spending bill that includes what Democrats call "human infrastructure" — Medicare expansion, national paid family leave, free college tuition, amnesty for some illegal immigrants, child tax credit expansion, and Green New Deal climate policies. These are the policies favored by progressive lawmakers.

The two bills together comprise almost the entirety of President Joe Biden's economic agenda.

Senate Democrats do not have Republican support for the $3.5 trillion spending bill, so they are trying to pass the it through a process called budget reconciliation, which allows them to circumvent a filibuster attempt by Republicans and pass the bill with a simple majority. But in order to pass the bill, all 50 members of the Democratic majority need to vote in the affirmative so that Vice President Kamala Harris can cast a tie-breaking vote in the 50-50 Senate.

Enter Sen. Joe Manchin (D-W.Va.). The moderate West Virginia Democrat issued a statement on Wednesday saying he has "serious concerns" about the size of the $3.5 trillion budget reconciliation package.

Manchin noted that Congress "has injected more than $5 trillion of stimulus into the American economy" since the beginning of the COVID-19 pandemic and that as a result, inflation is rising at rates unseen for decades.

"These are not indications of an economy that requires trillions in additional spending," Manchin said. "Every elected leader is chosen to make difficult decisions. Adding trillions of dollars more to nearly $29 trillion of national debt, without any consideration of the negative effects on our children and grandchildren, is one of those decisions that has become far too easy in Washington."

"Given the current state of the economic recovery, it is simply irresponsible to continue spending at levels more suited to respond to a Great Depression or Great Recession – not an economy that is on the verge of overheating," he continued. "More importantly, I firmly believe that continuing to spend at irresponsible levels puts at risk our nation's ability to respond to the unforeseen crises our country could face."

Manchin's vote is needed for the Democrats to pass the $3.5 trillion reconciliation bill. If he opposes the bill because it spends too much, he could kill it and progressive Democrats in the House will prevent the $1 trillion bipartisan infrastructure deal from passing there. House Democrats could also hold up the bipartisan infrastructure deal if they don't like potential changes or spending reductions to the reconciliation package Senate Democrats might make to secure Manchin's vote.

The Senate passed a budget resolution early Wednesday on a party-line vote that creates a framework for the $3.5 trillion reconciliation package, but the actual text of that legislation won't be released until the fall.

To get the $1 trillion bipartisan infrastructure deal to Biden's desk for his signature, Democrats will need to balance Manchin's desire for a modicum of fiscal responsibility with the spendthrift House progressives.