Even though the GOP is about to control both the White House and Congress, they’re still not free to do whatever they want. As they were reminded this week, congressional Republicans could still see their ambitions blocked by angry phone calls, or even a presidential tweet.

The House GOP’s failed attempt to gut the Office of Congressional Ethics this week provided an early glimpse into the hazards Republicans could encounter as they seek to push a sweeping policy agenda through Congress. Those include the very real threats a public backlash can pose to a party with unified control of Washington and the newfound power to pass legislation — and the possibility that Donald Trump could abandon their cause in the face of such a backlash.

The first of those lessons reared up after House Republicans, many of whom have felt aggrieved at what they’ve deemed an overzealous ethics watchdog in recent year, conducted a closed-door vote Monday night to advance a proposal that would have stripped the ethics office of many of its powers for the new Congress.

But as news of the vote broke, activists raised a ruckus, outrage spread across social media, and the controversy dominated the news Tuesday morning, the first day of the new Congress. Congressional offices were barraged by furious phone calls from their constituents criticizing the move. Trump eventually piled on, complaining on Twitter that the House GOP shouldn’t make this “their number one act and priority.” By midday, House Republicans reversed course.

Republicans are the governing party now — which means they’ll face much tougher public scrutiny

For the past eight years, divided government has inadvertently protected the House GOP from electoral consequences for many of their legislative actions. Even the unpopular GOP-driven shutdown of the federal government in 2013 was soon forgotten by the electorate, since a majority of the voters consistently disapproved of President Barack Obama’s job performance the following year, and voted accordingly.

But now Republicans are in charge, and their actions all of a sudden seem to matter, as Gregory Koger, a political science professor at the University of Miami, argues. “For the last six years, their organizing strategy has been to be a protest party, and all their actions have been interpreted not as actual governing but as protest,” he says. Now, “people will start evaluating their proposals as laws that will actually affect people’s lives.”

About two-thirds of Republicans in the House have never served under a Republican president, which means Republicans will have to figure out “what it means to be the party that controls the House, Senate, and the presidency for the first time in a while,” says Molly Reynolds, a Brookings Institute fellow on governance studies.

Health care reform is the most immediate and prominent example of that recalculation. Repealing Obamacare without an immediate replacement — a move favored by some in the GOP — could deprive about 20 million people of insurance and provoke an outcry from the Americans affected.

“It’s one thing to say, ‘oh yeah, I hate Obamacare and I really want to get rid of it,’” Koger says. “It’s another thing to contemplate stripping Medicaid away from millions of poor people without any real plan to take care of it.”

Another major challenge for the GOP: President Trump’s unpredictability

Trump is an even bigger potential hurdle — a GOP president who appears to have little inclination toward pure partisan loyalty and little love for some of Republicans’ traditional policy priorities.

Though there’s some dispute over whether the public backlash or Trump’s own critical tweets were more responsible for the House GOP’s reversal, Trump’s decision to tweet at all rather than simply silent on the issue suggests he’s willing to throw the House GOP under the bus to make himself look good. And when he remains so popular among Republican voters, a GOP Congress fearing primary challenges will worry about defying him.

As Tim Alberta wrote in an excellent recent profile piece, there are early signs that the Freedom Caucus — who had styled themselves as the House’s staunchest conservatives — are reinventing themselves as fervent Trump supporters, in many cases because they’ve concluded their base voters care far more about Trump than they do about free-market orthodoxy.

“The Republican Party is going through a bit of a metamorphosis with regards to its base,” says Joshua Huder, an expert on Congress from Georgetown’s Government Affairs Institute. “There’s a lot of uncertainty on the Hill right now. How much are these very conservative members of Congress going to cow to the views of their new president?”

Trump tweeted a cryptic warning to the GOP on Wednesday morning, writing that Republicans should be “careful” to ensure Democrats continue to “own the failed ObamaCare disaster.” While it’s not entirely clear what Trump meant by those tweet, during the campaign he seemed to be far more cautious about the political consequences of repeal than others in the GOP field were, repeatedly saying he wouldn’t let people “die on the streets” and that he’d replace Obamacare with “something terrific.”

So while remains unsettled for the new congressional GOP, the messy start to the new session points to some stormy days ahead. Governing is a whole lot more challenging than being in the opposition — and governing alongside President Trump could well prove to be very challenging indeed.