Last week marked the third historic Wednesday of a year that’s had only three Wednesdays: After the grave events of the storming of the Capitol on Jan. 6 and the first-ever second impeachment of a president on Jan. 13, the nation witnessed the always miraculous peaceful transfer of power when President Joe Biden was inaugurated on Jan. 20. But just hours after the pomp and circumstance of the morning’s ceremony, newly sworn-in Vice President Kamala Harris began the routine practice of presiding over the Senate and, in turn, swore in the body’s newest members: Alex Padilla of California and Jon Ossoff and Raphael Warnock of Georgia.

The country’s first female vice president swearing in the first Hispanic, Jewish and Black senators from their respective states is a remarkable moment worth celebrating, but more sweeping was the way that moment shifted the dynamics of power in the United States.

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New York’s Chuck Schumer is now Senate Majority Leader — but that may not mean much. With an even 50 Democrats and 50 Republicans now in the chamber, Schumer’s party really has the majority only when Harris gets to cast a tie-breaking vote. Since Democrats also control the House of Representatives and the Oval Office, that tie-breaking vote will undoubtedly shift the political balance of the nation to the left on some issues — but many of the more high-profile and controversial issues face less predictable fates.

The 117th Congress will be the year of the centrist. (Well, two years, technically.) There are enough moderate senators, lawmakers on both sides who’ve refused to toe the party line, that an even 50-50 split on legislation is unlikely to occur frequently. And if Schumer continues to hold the nuclear option over the heads of GOP senators, that threat should be enough to cow most Republicans into not filibustering excessively. Thus, many of Biden’s most triumphant victories in Congress, and his most crushing defeats, may come down to just a handful of key players — with a history of bucking their parties and a future of deciding policy that’ll affect 330 million Americans. Here’s who to watch for in coming sessions.

Sen. Joe Manchin, D-W.Va., is pictured at the U.S. Capitol Wednesday, Jan 29, 2020, in Washington. | Steve Helber, Associated Press

Joe Manchin

The West Virginia senator has sometimes been derided by others in his party as a “DINO” — a Democrat In Name Only. The conservative Democrat has certainly voted with his party many times, like to convict then-President Donald Trump last year and to protect Obamacare, but he’s bucked his party on issues like energy, police funding, Medicare for All, and crucially, the judiciary, vowing to block his colleagues from packing the Supreme Court. Manchin also squabbled publicly with Rep. Alexandria Ocasio-Cortez over progressive policies in what was perhaps the most high-profile case of infighting for a party that’s normally in tight lockstep.

Sen. Kyrsten Sinema, D-Ariz., is pictured on Capitol Hill in Washington, Tuesday, July 16, 2019. | Manuel Balce Ceneta, Associated Press

Kyrsten Sinema

Arizona’s senior Democrat has established herself as a freewheeling member of Congress who doesn’t much care about the “D” after her name. Behind Manchin, who she cites as an inspiration, she voted with Trump’s position the most of any Democrat who’s served a full term. The first openly bisexual member of Congress, Sinema’s conservative streak includes votes on immigration, the economy, foreign policy, the environment and telecommunications, but she aligns more closely with her party on gun control and social issues like abortion and LGBT rights.

Democratic former Colorado Gov. John Hickenlooper is pictured on Friday, Oct. 9, 2020, in Denver. | Hyoung Chang, The Denver Post via Associated Press

John Hickenlooper

Rounding out the list of Democrats to watch is Colorado’s newest senator. The only freshman on this list, he doesn’t have much of a voting record yet, but in the early days of the 2020 presidential race he campaigned as perhaps the most moderate candidate for the Democratic nomination, criticizing Medicare for All and the Green New Deal and warning against socialism. As governor of the state he now represents in D.C., Hickenlooper supported fracking and described himself as a “fiscal conservative” who doesn’t “think the government needs to be bigger,” but he’s hewn more closely to his party on gun control and marijuana policies.

In this June 23, 2020, file photo Sen. Lisa Murkowski, R-Alaska, is pictured on Capitol Hill in Washington. | Michael Reynolds, pool via Associated Press

Lisa Murkowski

Alaska’s senior Republican is a true wildcard, having voted both with and against her party on a huge range of issues. Notably, she usually sides with Democrats on abortion and LGBT issues, but she’s consistently conservative on gun control and taxes. Murkowski has been the Republican most open to the idea of convicting Trump in his upcoming Senate trial, though she hasn’t yet vowed to vote one way or the other.

Sen. Mitt Romney, R-Utah, is pictured on Capitol Hill, Wednesday, Sept. 16, 2020, in Washington. | Manuel Balce Ceneta, Associated Press

Mitt Romney

The high-profile Republican from Utah has had his Senate career marked mostly by public feuds with Trump, becoming the only Republican to vote to convict him in last year’s impeachment trial (though he hasn’t signaled how he’ll vote in the second trial). That said, he’s voted with the president’s conservative positions 75% of the time, making him the most reliably partisan lawmaker on this list. With Trump now out of the picture, it remains to be seen whether Romney’s ire with pro-Trump Republicans and his sympathy for Biden will influence how he votes.

Sen. Susan Collins, R-Maine, is pictured at the Capitol in Washington, Sunday, Jan. 3, 2021. | Kevin Dietsch, pool via Associated Press

Susan Collins

The final centrist on the list is the Senate’s most senior Republican woman. The Maine lawmaker voted frequently with former President Barack Obama’s positions during his administration, but she also voted frequently with her party during Trump’s administration. Like Murkowski, she tends to be fiscally conservative but socially liberal. Collins’ tendency to vote with her party has changed drastically year to year over the course of her two decades in the Senate, so it’s difficult to predict how much of a team player she’ll be under a Biden administration.

Then there’s Senate Minority Leader Mitch McConnell. He and Biden have a working relationship that’s spanned almost four decades, and Biden is a creature of the Senate if there ever was one. Their history, and their ability to negotiate with each other, could be the cornerstone of many of the bills that ultimately get passed. Plus, if the threat of abolishing the filibuster continues to loom large, McConnell can exert influence over his conference to ensure his more fiery colleagues don’t abuse — and lose — the procedure.

Regardless of whether the legislative filibuster goes the way of the judicial one, the lawmakers listed here have their path carved out for them: becoming some of the most powerful men and women in America.