President Trump with Senate Majority Leader Mitch McConnell (R-Ky.) Vice President Pence and House Speaker Paul D. Ryan (R-Wis.) at the White House Wednesday to celebrate passage of the GOP tax cut bill. (Jabin Botsford/Washington, D.C.)

A day after standing shoulder to shoulder at the White House to celebrate passage of a major tax bill, tensions emerged among Republicans on Thursday over what part of their agenda to tackle next as the party tries to capi­tal­ize on the biggest legislative success of the Trump presidency.

For weeks, House Speaker Paul D. Ryan (R-Wis.) and other conservatives have been trying to build momentum for reining in spending on Medicare, Medicaid and other safety-net programs, long-standing goals that catapulted Ryan to prominence in the GOP.

But Senate Majority Leader Mitch McConnell (R-Ky.) threw cold water on those ambitions Thursday, saying he would rather focus on an issue with potential for bipartisan appeal: an infrastructure initiative that spurs new spending on the nation's ailing roads, bridges, airports and waterways.

"I don't think, as a practical matter in the Senate, we can do entitlement reform without bipartisan agreement," McConnell, who will have only 51 Republicans in his chamber come January, told The Washington Post. "And you can fill in the blanks. I mean, it applies to entitlements in general — Medicare, Social Security, welfare — they're so doubled down on that, I'm not going to devote floor time to something that has no Democratic support."

With the euphoria of the passage of a major tax bill still running high, the discord over what's next reflects a tough reality for the GOP: Heading into an election year, nothing being contemplated will be easy.

Aides to President Trump say the White House has yet to finalize an agenda for the new year but is interested in pushing both an infrastructure plan — which could be rolled out as early as mid-January — as well as more targeted revisions to welfare programs, such as cash assistance and food stamps, than Ryan has floated.

A senior administration official told reporters at a briefing Thursday that Trump, Ryan and McConnell plan to meet on welfare reform in early January "to make sure everyone is on the same page."

But some Republicans worry about taking aim at any safety-net programs on the heels of passing a $1.5 trillion tax bill that independent analysts say does far more for corporations and the wealthy than the middle class.

"I think the optics are terrible," said John McKager "Mac" Stipanovich, a longtime Florida-based GOP consultant. "At least at this point, the Democrats are winning the argument that the tax cuts primarily benefit the wealthy and big business. To come in right behind that and start whacking the poor, the working poor, will not serve Republicans well in 2018."

Democratic operatives, meanwhile, are salivating at the prospect of Republicans targeting programs that benefit working-class voters, a constituency that Democrats are trying to bring back into the fold after Trump's victory last year.

"Trump and Ryan are acting like people who aren't going to be around to deal with the consequences of their actions," said Mary Anne Marsh, a Boston-based Democratic consultant. "That's the only explanation I can think of."

Building consensus on infrastructure legislation won't be easy, either.

The plan being developed by the White House would reward states and localities that raise taxes or other new revenue to pay for new projects. It's an approach that some members of both parties are likely to find objectionable for different reasons, and it's unclear how Washington would pay its share of the $1 trillion plan envisioned by the White House.

Ryan has been signaling for weeks that, after tax cuts, the GOP should turn its attention to the spending side of the ledger.

"We're going to have to get back next year at entitlement reform, which is how you tackle the debt and the deficit," Ryan said in a radio interview earlier this month, calling Medicare and Medicaid the "big drivers of debt." In the same interview with conservative radio host Ross Kaminsky, Ryan said he was working to convince Trump of the wisdom of his approach.

As a candidate, Trump vowed not to cut Medicare or Social Security, a promise that aides said in recent days is still operative, even as he looks to make changes to other safety-net programs. Much of the White House focus, an aide said, would be on giving states more flexibility to manage those programs and to impose new work requirements and other conditions for public assistance.

In a television interview Wednesday on ABC, Ryan seemed to retreat slightly, saying he wasn't likely to seek broad reforms to Medicare in 2018 but would focus on narrower issues, such as reining in payments to health-care providers that he said are "getting overpaid."

On Thursday, hours after McConnell, speaking at a breakfast sponsored by Axios, expressed first doubts about tackling entitlement programs in 2018, Ryan declined to answer questions from reporters.

"Take this moment, just enjoy this moment," Ryan said, literally brushing away reporters with his right hand as he walked out of a room where Republicans took part in a formal ceremony to send the tax legislation to Trump for his signature.

A Ryan aide said House Republicans plan to hash out their 2018 agenda at a retreat planned in late January.

House Minority Leader Nancy Pelosi (D-Calif.) on Thursday dismissed Republican talk of entitlement reform as "part of the 'Starve the Beast' values system that Republicans have."

"They do not believe in governance, so any public role in the health or well-being of the American people is on their hit list," she told reporters, adding later that Democrats will not let Republicans "use Medicare, Medicaid and Social Security as an ATM" to pay for the tax cuts.

In the Senate, 60 votes are needed to cut off filibusters on most bills — most upcoming initiatives will require some Democratic support to move forward.

Many GOP lawmakers in both chambers agreed with McConnell that there is little urgency to make changes to social safety-net programs in the coming year.

"It's hard in the Senate. I know the House has made it a priority, and there are a lot of us who'd like to do it," Sen. John Thune (R-S.D.) said. But, he said, "it's a hard sell to some of our members. I think the leader is just reflecting what the political reality is."

Rep. Tom Reed (R-N.Y.), a member of the House Ways and Means Committee, said any revamp of entitlements makes more sense as "a longer-term conversation."

"It needs to get done sooner rather than later, but all the reports say that the problems are a decade out," he said.

Some Republicans, such as Sen. Pat Roberts (Kan.), expressed concern about making any safety-net program a target next year.

As chairman of the Senate Agriculture Committee, Roberts will play a big role in 2018 as Congress begins debating a new Farm Bill, legislation that sets agriculture policy but mostly funds food stamps, school lunch programs and other anti-poverty programs.

"This talk about welfare reform across the board, or Social Security or this, that and the other, I think is very premature," he added. "We're going to have our plate full doing other things we have to do."

Still, Trump boosters said they see hope for buy-in on narrower reforms, such as empowering states to set more rules for receiving food stamps and other benefits.

"I think that to the average voter it plays quite well," said Barry Bennett, a GOP consultant who advised Trump during the general election. "Everyone wants to make sure people get the help they need but need the help they get."

Karoun Demirjian, Erica Werner and Michael Scherer contributed to this report.