Sinema's exit from the Democratic Party could hand Manchin back some power over Biden's agenda

• Sen. Kyrsten Sinema is leaving the Democratic Party to become an independent.
• It changes the balance of power in the Senate, though Sinema sought to downplay the extent.

Only days ago, the mood among Democrats was jubilant.

The victory of incumbent Sen. Raphael Warnock in the Georgia runoff strengthened their position in the Senate, giving them a precious 51-49 outright majority.

But Sen. Kyrsten Sinema's announcement early Friday that she is leaving the Democratic Party to become an independent soured that victory, and again altered the balance of power.

For the first two years of President Joe Biden's term in office, the Democrats had the narrowest of majorities in the 50-50 divided Senate owing to Vice President Kamala Harris' tie-breaker vote.

With control of the Senate on a knife-edge, centrist Democrats Sinema and Sen. Joe Manchin of West Virginia wielded outsized influence, with the power to force sweeping changes to legislation under threat of withholding their support.

They could even torpedo bills outright. The party needed the support of all 50 of senators to pass legislation. Though that group already included two independents — Sen. Bernie Sanders and Sen. Angus King — it was signed-up Democrats, Sinema and Manchin, who caused the most trouble.

The two senators, sometimes individually and sometimes together, refused to back key Democratic initiatives on the economy, climate change, corporate taxation, and gun control, frustrating the party and blocking Biden's drive to enact core election pledges.

Both Manchin and Sinema opposed reforming the filibuster rule, which would've enabled the Democrats to sidestep GOP opposition and pass voting rights reform with a simple majority vote.

With Warnock's victory Tuesday, the Democrats appeared to have enhanced their position in the Senate by one seat, giving them crucial breathing space, and mitigating Manchin and Sinema's influence.

But with Sinema's announcement, the Democratic majority in the Senate has been cut back, and Manchin's influence apparently enhanced.

But there are key differences between the situation now and that which existed for Democrats before the midterms, and Sinema herself insisted that the likelihood of sweeping change in the wake of her shift was low.

The overall balance of power in Congress shifted in the midterm election, with the Republicans now narrowly controlling the House. This means that chamber can now block the White House's legislative packages, regardless of dynamics in the Senate, meaning that a renegade senator is now only one of several potential holdups.

Sinema's new position is ambiguous. In an interview with Politico explaining the move, she said she would vote in a similar way to before and would not coordinate either with Republicans or Democrats (King and Sanders do coordinate with Democrats even though they are independent).

Sinema also said she wanted to keep her committee seats, which relies on maintaining ties with Democrats. Senate Majority Leader Chuck Schumer said in a statement Friday that Sinema leaving the party wouldn't impact its new majority on committees.

"Becoming an independent won't change my work in the Senate; my service to Arizona remains the same," she said in an op-ed announcing her departure from the Democratic Party.