CNN  — 

President Joe Biden and fellow Democrats are racing to prove a counterintuitive lesson: with congressional majorities, sometimes less can be more.

The new President holds a slim House advantage and even slimmer Senate edge – 51-50, and then only if he holds every Democrat and allows Vice President Kamala Harris to break ties. Yet passage of the Senate version this weekend keeps party leaders on track to overcome rock-solid Republican opposition and put a $1.9 trillion Covid relief bill on Biden’s desk within days, though even some prominent Democratic economists consider it too costly.

By the standards of recent history, that would represent an audacious feat. Every Democratic president in the last half century has taken office with at least 57 Democratic senators, and still struggled to achieve top objectives.

In part, Biden’s shot at pulling it off reflects the ideological cohesion that the decades-long realignment of both parties has produced. But it also represents the pressure to unite that small majorities impose on disparate partisans who know their common window to succeed is narrow.

“There’s an emotional camaraderie – we’re all in this together,” said Rahm Emanuel, a former House member who become President Barack Obama’s first White House chief of staff. “When there’s only 50 votes, it’s really hard to hide.”

Emanuel lived through first-year challenges as an aide to two Democratic presidents. After defections by six Senate Democrats, President Bill Clinton needed a tie-breaking vote from Vice President Al Gore to enact his economic program in 1993. Sixteen years later, recalcitrance from conservative Democrats shrank Obama’s economic stimulus plan, constrained his health care program and killed his climate change bill.

The defections they faced look small compared with what Democratic predecessors had to overcome. President Jimmy Carter had 62 Senate Democratsbut only modest legislative success, needing major Republican help to ratify the Panama Canal Treaty. Only an alliance with Senate Republican leader Everett Dirksen allowed President Lyndon Johnson, with 68 Senate Democrats, to overcome a filibuster by Southerners in his own party to pass the 1965 Voting Rights Act.

Johnson’s feat, echoing over decades, had the ironic effect of reducing future opportunities for bipartisan cooperation. Anti-civil rights Southern conservatives abandoned the Democratic Party, while liberal Northeastern and Midwestern Republicans did the opposite, forging today’s more ideologically homogenous and polarized parties.

The risks of pushing too hard

Pushing the few remaining iconoclasts too hard can backfire. Squeezed during debate over President George W. Bush’s tax cuts in 2001, then-Sen. Jim Jeffords of Vermont quit the GOP and flipped a 50-50 Senate to Democratic control.

But Sen. Joe Manchin of West Virginia shows no such intention, notwithstanding his decision to sink Neera Tanden’s nomination as budget director and hold up Senate action on Covid relief on Friday over unemployment insurance provisions.

Senate Democrats only tinkered with Biden’s Covid relief bill. They maintained its overall spending level and preserved its income thresholds for full $1,400 relief checks, while phasing out the checks more quickly above those thresholds.

After hours of delay, Manchin accepted only minor changes on the extension of unemployment insurance. The loss of the $15-an-hour minimum wage provision – felled by a Senate parliamentarian’s ruling that Democrats lacked the votes to overturn – does not appear to endanger the relief bill’s final passage in the House.

It helps that Biden’s first major initiative lavishes spending on key constituencies without raising anyone’s taxes. Economic issues unify Democrats more than cultural concerns.

So does shared recognition that their opportunity for action may be brief. The history of midterm elections suggests Republicans have a strong chance to regain control of Congress next year.

That strengthens internal arguments by Democratic leaders to aim high, using legislative procedures that circumvent a Republican filibuster.

“The question,” notes George Washington University congressional scholar Sarah Binder, “is how long can Biden hold that together.”

The challenge after the relief bill

One indication will come with forthcoming legislation embodying Biden’s call to “Build Back Better” after Covid. Potential provisions range from infrastructure to immigration to health care; the price tag could reach $3 trillion.

With no prospect of significant Republican support, Democrats could enact such a mammoth bill only by rallying behind the same no-filibuster budget procedures used for Biden’s Covid relief plan. Whether they try will test the party’s shared appetite for boldness.

The stiffest solidarity test involves the intensifying Democratic drive to change the Senate itself. Manchin and Sen. Kyrsten Sinema of Arizona, burnishing reputations for bipartisan accommodation, have broken with colleagues by pledging to preserve filibuster rules that allow a unified Senate minority to block all non-budgetary legislation.

Other Democrats insist, with increasing urgency since the deadly US Capitol insurrection, that the party cannot afford to let Republicans block action on one issue above all others. Alarmed by Biden’s victory – the seventh time in the last eight presidential contests that America’s diversifying electorate has cast more votes for the Democratic nominee – Republican state legislators have moved aggressively to curb access to the ballot.

Last week, House Democrats passed legislation to halt those rollbacks by setting federal registration and voting standards. Only unifying to scrap the filibuster would allow Senate Democrats to follow suit in protecting voting rights, the very issue Johnson needed bipartisan support to advance in 1965.

Democratic proponents can only hope that invoking the higher purpose at stake – safeguarding the democracy enshrined in the US Constitution – will ultimately persuade Manchin and Sinema to go along.

“We’re going to have an intense conversation in our caucus,” says Democratic Sen. Jeff Merkley of Oregon, lead Senate sponsor of voting rights legislation. “It’s too early to know how we will solve this. I think we’ll find a way.”