Republicans may have full control in Washington, but the Senate remains the Senate, which means it’s the place where rank-and-file Democrats and Republicans retain the most clout and potential for influence. Here are the key senators from outside of the top echelons of the leadership structures to watch as the 115th Congress gets underway.

REPUBLICANS

Maine’s Susan Collins, The First Defector

The moderate from Maine will be the first person to watch on any contentious votes, particularly on budget reconciliation votes that aim to repeal parts of the 2010 health care law. She has, for instance, been among the small number of Republicans opposing efforts to tie the GOP health care plans to stopping federal funding of Planned Parenthood.

Arizona’s Jeff Flake, Primary Target

Before the 2016 election, Flake made headlines for sparring with Donald Trump, making him perhaps the GOP senator most vulnerable to a primary challenge from supporters of the president-elect. But the fiscal conservative is not going to run away from principles. Flake is an original co-sponsor of a bill to shield recipients of President Barack Obama’s immigration policy that deferred action on immigrants brought to the United States illegally when they were young children from possible action by Trump.

Nevada’s Dean Heller, Republican at Risk?

Heller considered a bid for governor, but has decided to seek another term in the Senate instead. Silver State Democrats do not yet have a challenger, and the conventional wisdom is that the opposition bench is rather thin. But on a 2018 map that overwhelmingly favors the GOP, Heller might be the most at risk of losing in this swing state if the Trump administration proves unpopular.

Arizona’s John McCain and South Carolina’s Lindsey Graham, Maverick Amigos

McCain and Graham, who are never shy about their opinions, have been the most aggressive Republicans in calling for action against Russia over the country’s efforts to influence the 2016 presidential election. With the push for a select committee stopped in its tracks, the Armed Services Committee, which McCain chairs and of which Graham is a senior member, may be ground zero for any investigation of cybersecurity vulnerabilities.

Georgia’s David Perdue, Ally of the Administration

The former businessman has been a vocal advocate for the president-elect and a visitor to Trump Tower. He is fashioning himself as an intermediary between the administration and the Senate.

DEMOCRATS:

West Virginia’s Joe Manchin III, The Democratic Link

Manchin is one of the most vulnerable Democrats heading into 2018, running for re-election in a state the president-elect won by 42 points. Manchin has said he plans to be the Senate Democrats’ liaison with the Trump White House, noting earlier this month that he has exchanged cell phone numbers with the president-elect and incoming Vice President Mike Pence. He’ll be one of the Democrats to watch on close votes as someone who could side with Republicans.

New Jersey’s Cory Booker, Eye on 2020

Booker has been rumored to be a potential candidate for president in 2020. After keeping a relatively low profile when he first came to the Senate, he’ll be one to watch on whether he takes a more vocal role, especially in opposition to Trump. Booker could also be looking to bolster his foreign policy credentials with his new seat on the Foreign Relations Committee.

Maryland’s Chris Van Hollen, Not Your Average Freshman

Van Hollen is an unusual first-term senator, quickly taking a spot in leadership as head of the Democratic Senatorial Campaign Committee, which oversees the campaigns for the 25 Democrats defending their seats in 2018. Van Hollen is also well known around the Capitol due to his time in the House, where he served as the top Democrat on the Budget Committee and previously as head of the Democratic Congressional Campaign Committee and as assistant to the speaker under Nancy Pelosi.

California’s Kamala Harris, Rising Star

The former California attorney general is a new senator to watch, and has also been listed as a potential candidate for president. Some of her top priorities include overhauling the criminal justice system, addressing climate change, and resolving immigration issues. She will also be earning more national security experience through her seat on the Intelligence Committee. The panel will likely be in the spotlight following its announcement that it would investigate Russian interference in the 2016 presidential election.

Maine’s Angus King, Moderate Dealmaker

The former governor is an independent but caucuses with the Democrats, and he will be a moderate to watch as lawmakers work for bipartisan consensus. First elected in 2012, King could face a 2018 challenge from the right, but Inside Elections with Nathan L. Gonzalez rates his race as a Solid Democratic contest. King’s seats on the Armed Services and Intelligence committees also make him a key lawmaker as senators look to address issues like cybersecurity.

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