For the first time since 2010, Washington will be a one-party town and the Republicans on the cusp of controlling the presidency and both houses of Congress are already preparing an ambitious agenda.

After six years of partisan gridlock, GOP leaders of the 115th Congress begin a new session Tuesday eager to roll back much of President Barack Obama’s crowning achievements, including the Affordable Care Act, as well as advance their own policies, like tax reform, that have been mothballed for years. And they will be navigating the new terrain with a mix of familiar faces and new names on the scene.

In the leadership, House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell will continue to be instrumental players who will guide their members, negotiate with incoming President Donald Trump and dictate the agenda. They will continue to be two of the most powerful people in Washington.

But while their goals are likely to be the same, the challenges they face will be different. Ryan will have to keep his diverse caucus united to pass legislation and McConnell, with a majority of 52 senators, will likely need the support of eight Democrats to pass anything controversial or significant. That means legislation coming out of the Senate must have at least some bipartisan appeal, and in order for it to pass the House, the legislation will have to be conservative enough to appease Ryan's most right-leaning members.

The most critical figure in the opposition is a familiar face in a new role — Senate Minority Leader Chuck Schumer of New York. He replaces retired Sen. Harry Reid of Nevada who kept Senate Democrats in line, defended his members and put up fierce opposition to any Republican proposal or tactic he thought was unpalatable. Schumer has indicated a more conciliatory approach, vowing to work with Trump on areas where he thinks they could agree like on jobs and infrastructure.

Because senators have expanded the use of the filibuster, it often takes the support of 60 senators to pass legislation. Schumer’s 48-member caucus can easily block bills, but it’s going to be up to Schumer to try and keep his caucus from defecting to vote with Republican measures if they are going to truly block the GOP agenda.

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Regardless of his style, his success will be measured on the results. Schumer will drive the Democrats’ messaging to voters, which will largely dictate how effective Democrats are over the next two years.

Outside of leadership, these lawmakers are expected to be critical players on Capitol Hill over the next two years:

REPUBLICANS:

Sen. Susan Collins, R-Maine: Collins is a moderate Republican who Democrats could turn to for help in blocking some components of the Republican agenda. While Democrats won’t dictate the agenda on the Senate floor, they could offer amendments to alter legislation, and getting the support of Collins could help Democrats win small victories.

Sen. Dean Heller, R-Nevada: Of all the 33 senate seats up in 2018, Heller is the only Republican up for re-election in a state that Trump lost. Heller will have to spend the next two years navigating Trump, his Republican leadership, and his prospects for re-election. If Trump is unpopular, especially in Nevada, Heller could become a critical swing vote for Senate Democrats.

Rep. Mark Meadows, R-North Carolina: Meadows is the new leader of the conservative Freedom Caucus, which consists of about 40 Republicans who demand a limited government and prefer a uncompromising approach. As their leader, Meadows will be one of the most influential members of the House. Speaker Ryan will have to ensure that he has the support of Meadows and his fellow Freedom Caucus members if he is to pass any legislation without having to gain the support of Democrats.

As chair of the House Oversight Committee, Rep. Jason Chaffetz will be tasked with deciding how to handle potential legal or ethical conflicts of the incoming administration. Cliff Owen / AP

Rep. Jason Chaffetz, R-Utah: The head of the House Oversight and Government Reform Committee has been bullish on investigating the Obama administration, including Democratic presidential nominee Hillary Clinton. The question is if he will give the same due diligence to oversight during a Trump administration. If he decides to conduct a thorough vetting of Trump and his appointees, he could become one of the most interesting and closely watched members to determine the areas where Republicans are not pleased with their president.

Rep. Tom Price, R-Georgia: Price has been tapped as Trump’s secretary of health and human services, but until he is confirmed by the Senate, he will play a central role in the first few weeks of Congress in January. As head of the Budget Committee, he will oversee the budget that includes repeals of many aspects of the Affordable Care Act. He wrote the legislation, called Reconciliation, passed in 2015 to repeal much of Obamacare and it is likely to be the blueprint for repeal in 2017. As HHS secretary, he is expected to work very closely with members on a replacement of the health care law.

Sen. John McCain, R-Arizona: McCain is an old name in the Senate, but the moniker “maverick” he adopted during his 2008 presidential run is likely to make a comeback. He is no fan of Trump, and as head of the Senate Armed Services Committee, he and his committee could cause problems for Trump's foreign policy, including Trump’s forgiving stance toward Russia.

DEMOCRATS

Sen. Bernie Sanders, I-Vermont: Fresh off a presidential bid where he amassed a large and passionate following, his new role in Democratic leadership is outreach liaison. Sanders will be instrumental in determining if Democrats can effectively message to white working class voters — voters Trump won overwhelmingly. This fiercely independent senator will now be responsible for seeing if Democrats can make any gains with these voters in two years.

Sen. Joe Manchin, D-West Virginia: Manchin is up for re-election in 2018 in one of the ten states Trump won that Democrats must now defend. Manchin, who describes himself as fiscally conservative and socially compassionate, could defect from Democrats on some issues and play a critical role in Republicans’ strategy to pass their agenda.

Sen. Joe Donnelly, D-Indiana: Donnelly is another senator, like Manchin, who is up for re-election in a Trump-won state. The difference between Manchin and Donnelly, however, is that Donnelly is considered the most endangered incumbent. Indiana voted for Trump by 19 points, which means Donnelly, who is serving his first term, will have to make some very tough decisions on how he votes on the Senate floor. All eyes will be on him to see if Republicans can pass a bill or Democrats will be able to block it.

Sen. Claire McCaskill, D-Missouri: Also from a Trump-won state and up for re-election in 2018, McCaskill is a solid Democrat, but she has been known to exert her independence at times. If her re-election is looking more difficult as 2018 nears, McCaskill could be a good place to look when Republicans need to reach across the aisle.

Rep. Joe Crowley, D-New York: Crowley was elected to House leadership as chair of the Democratic caucus. While he’s been in office since 1999, Crowley is seen as the bridge between Nancy Pelosi and her leadership team, which came under pressure to step down by some members, including younger and newer members. He’s ambitious but well-liked, and he could help to steer Democratic policy in a direction that younger members would prefer.