(CNN) Congressional Republicans return to work this week thrilled they passed a major tax reform bill before Christmas but uncertain and divided about what they will do in 2018, a critical election year.

Entering their second year of unified control of Washington, GOP lawmakers have more questions than answers about what they can accomplish, with divergent ideas over the policy agenda and political path to the midterms. At the heart of it all is a significant rift between the top leaders in the House and Senate about whether to tackle entitlement reform, a longstanding GOP priority.

On key issues like health care, immigration and reining in debt from safety net programs, congressional Republicans are split, just when a more unified message might help convince voters to keep them in power.

History shows that in the first midterm election of a new presidency, the president's party usually suffers.

GOP leaders are anxious to escape that fate.

Divisions over entitlement reform and another Obamacare repeal

Senate Majority Leader Mitch McConnell, R-Kentucky, has provided few details of what he will put on the floor this year but he has made clear he doesn't plan to force controversial entitlement changes through his narrowly divided chamber or make another attempt at a broad repeal of the Affordable Care Act, which failed last summer.

"I wish them well," McConnell said bluntly when asked about the continued effort by Sen. Lindsey Graham, R-South Carolina, to push forward on his repeal proposal, which fell short of the necessary votes in September. McConnell added that he wanted Graham, and the bill's co-sponsor, Sen. Bill Cassidy, R-Louisiana, to continue working to gather support for their proposal. But McConnell noted that without the votes — and there's currently no sense among top GOP aides that the votes to advance the proposal exist — the top Senate Republican is limited in what he can do.

McConnell would also have to decide, likely soon, to push forward on another budget reconciliation effort in order to provide a vehicle in which to move a new repeal effort with a simple majority vote. At this point, there are no signs of any preparation for such an effort, aides say.

That bothers many House Republicans who pushed through an Obamacare repeal bill last year only to see it falter in the Senate.

JUST WATCHED Carpenter: GOP on Hill 'have made peace w Obamacare' Replay More Videos ... MUST WATCH Carpenter: GOP on Hill 'have made peace w Obamacare' 04:54

"There is some debate between Republicans, even in the House, and certainly Mitch McConnell," said Rep. Mark Walker of North Carolina, who chairs the conservative Republican Study Committee. "But that's a promise we have continued to make."

The first order of business when the Senate reconvenes Wednesday will be to swear in Alabama Democrat Doug Jones, who will slice into the GOP's already slim advantage.

"51-49," McConnell lamented recently, "is a pretty tight majority."

The reality of the Senate math means other Republicans -- including the powerful House speaker, Paul Ryan of Wisconsin -- may need to curb their ambitious policy goals because any substantial legislation will need at least nine Democrats to get the 60-vote supermajority required to pass.

JUST WATCHED Ryan says GOP will tackle entitlement reform Replay More Videos ... MUST WATCH Ryan says GOP will tackle entitlement reform 00:37

Ryan's enthusiasm to reform entitlements, something he's pushed to do for many years, was on display in a recent Fox News interview.

"We're going to take on welfare reform, which is another big entitlement program, where we're basically paying people -- able-bodied people -- not to work," Ryan said. "It is the perfect time to do welfare reform."

But it's unlikely many Democrats would back such changes, even if Trump makes a big push to sell the reforms. Even Democrats up for re-election in states Trump won -- who might see a political advantage to working with him -- showed only modest interest in his agenda during his first year in the White House.

That is a reality McConnell coldly pointed to when he rejected Ryan's push for an entitlement overhaul.

"I think that Democrats will not be interested in entitlement reform," McConnell told Axios. "So I would not expect to see that on the agenda."

Asked about the split, an aide to the speaker pointed to a meeting scheduled for next weekend at Camp David, when Ryan, McConnell and Trump will hash out the legislative agenda.

Another key meeting this week is expected to take place Wednesday, when White House chief of staff John Kelly plans to go to Capitol Hill to discuss spending issues with Ryan, McConnell and the top two Democratic leaders, House Minority Leader Nancy Pelosi of California and Senate Minority Leader Chuck Schumer of New York, according to two sources.

Sen. Thom Tillis, R-North Carolina, who flew to DC last week to preside over a short pro forma session of the Senate, said he hopes Trump uses the bully pulpit this year to convince centrist Democrats to support entitlement reform and other GOP goals.

"The speaker does a good job understanding where his votes are over there. The leader does a good job over here. The reality is a lot of what we need to do requires 60 votes," Tillis said. "So maybe the question is, how can the President get to people on the other side to produce a positive result?"

"Entitlement reform is not cutting, as everyone says it is," he added. "It's trying to get it on a sound fiscal footing."

Obamacare fixes, Dreamers, infrastructure

Conservatives in the House and centrist Republicans in the Senate are also split on whether to pass reforms to stabilize Obamacare or to allow young immigrants here illegally who were brought to the US as children to stay permanently.

They will work to narrow those differences when they hold legislative retreats at the end of the month.

Trump will give his State of the Union address January 30, a critical opportunity to sell the nation on the GOP's election-year agenda. Among other items, the former real estate developer plans to push Congress to back one of his top campaign pledges: spending billions to improve bridges, roads and other infrastructure projects across the country.

While some deficit hawks in the GOP are wary of spending hundreds of billions of dollars on infrastructure, many have said they are open to Trump's emerging proposal because it provides incentives and financing to states and localities, which will then pay for the bulk of the expected trillion-dollar plan.

Democrats are also divided on the plan. While most advocate a boost in federal dollars for infrastructure, which they think will create jobs and improve communities, many want a more robust package than the administration has outlined. And they fear Republicans will push spending cuts elsewhere in the budget to offset new infrastructure funding.

Democrats also worry that Trump is moving on a plan -- which will need their support to be approved, at least in the Senate -- without any bipartisan talks, a possible harbinger of bad things to come this year.

Leftover business

Before they can turn to an infrastructure bill and other 2018 priorities, lawmakers must resolve several important but difficult issues left over from 2017, including a long-term government spending plan, disaster aid and those possible reforms to Obamacare and immigration.

GOP leaders desperately wanted to complete those issues last year but couldn't as they focused primarily on passing tax reform, their most significant legislative victory of this session of Congress.

The government is set to run out of money January 19. Negotiators have worked to strike a deal that would raise budget caps on defense and domestic programs and allow a bill to pass that would fund the government through September, the end of the fiscal year. If they can't reach a deal on caps, government funding might have to limp forward on a series of stopgap measures to prevent a shutdown.

Those short-term bills also could become key political leverage points for House conservatives, who may tie their support to other fiscal demands, and for Democrats, who are insisting that controversial immigration legislation be attached.

Bipartisan lawmakers have negotiated legislation for months that would, as many Democrats want, formalize former President Barack Obama's executive order and allow the so-called Dreamers to legally stay in the US in exchange for Republican demands for beefed-up border security. While Democrats are pushing to have an agreement attached to any spending bill that must be approved by January 19, Republicans want to delay action until closer to a March deadline set by Trump to resolve the issue.

Leaders must also decide what to do with two bipartisan Senate bills related to stabilizing Obamacare's individual insurance market. They are being championed by Sen. Susan Collins, R-Maine, who was promised votes on the bills as part of her support for the just-passed tax measure.

She had wanted the votes before the tax bill was finalized but agreed to wait after negotiations over year-end fiscal issues collapsed before Christmas.

"There is every reason to believe that these important provisions can and will be delivered as part of a bipartisan agreement," Collins said in a statement at the time. "And Majority Leader McConnell has told us that he will uphold his commitment to schedule and support the legislation."

Going into 2018, Collins is arguably the most powerful Republican in Washington. A centrist from a purple state, she could decide the fate of many bills. It's one of the reasons McConnell worked so hard to assuage her concerns about the tax legislation.

But that doesn't mean she will necessarily get her way on the Obamacare bills. There is steep opposition from conservatives in the House to passing anything that bolsters Obamacare. To get the bills through the House, Trump may need to lobby those conservatives, or leaders might need to attach the legislation to another must-pass bill -- like a government funding measure -- and pass it over the objections of conservatives.

It's unclear how the issue will play out.

Also in the new year, Congress must agree to fund children's health insurance, raise the debt ceiling and reach an agreement to reauthorize the Foreign Intelligence Surveillance Act, a key tool for US intelligence agencies that allows them to track suspected terrorists. The issue divides security hawks, who want tough tools to go after bad guys, and libertarians, who fear the government could use it to unlawfully surveil Americans.

Lawmakers have also vowed to look inward to improve the way Congress deals with sexual harassment on Capitol Hill. They are considering legislation to reform the complaint process on the Hill and boost transparency around the sensitive issue. One leading proposal would also require members and staff to go through mandatory training every year, and would seek to give victims and whistleblowers more support.

Personnel changes

In addition to a new senator from Alabama, Vice President Mike Pence is expected to swear in a new senator from Minnesota. Former Lt. Gov. Tina Smith, a Democrat, will replace Democratic Sen. Al Franken, who resigned in disgrace after he was accused of groping women.

Also, Sen. Orrin Hatch of Utah, the longest-serving GOP senator, is set to announce as early as this week if he will run for re-election this year. The chairman of the Finance Committee, he played a critical role in passing the most significant tax overhaul in decades. If he leaves, it could open the seat to Mitt Romney, the former GOP presidential nominee and fierce critic of Trump.

It's really all about the midterms

Policy differences aside, arguably the most important objective for congressional Republicans this year will be to maintain control of the House and Senate. If Democrats seize either chamber in November, the GOP's legislative agenda would be imperiled. But perhaps more significant for Trump, Democrats would have the power to investigate the President -- and possibly impeach him -- over alleged collusion with Russia and his alleged efforts to obstruct a Justice Department investigation into the Russia matter.

With Jones' unexpected win in deep-red Alabama and a spate of GOP retirements in the House, both chambers are considered in play.

Republicans know it and fear it.

"We took a giant step in the right direction passing tax reform," Rep. Tom Cole, R-Oklahoma, said in an interview with CNN's Poppy Harlow. "When the American people give a majority, they expect you to do something with it. But look, no one's had a good midterm since 2002, and I think you're whistling past the political graveyard if you think you'll escape that challenging environment."