President Trump is undermining voter support for the Tax Cuts and Jobs Act with erratic messaging after it was gaining popularity, alarming Republicans counting on the law to save the party’s vulnerable House majority.

Senior Republicans are declining to publicly finger Trump for the heralded tax overhaul’s sagging approval ratings. Views of the law steadily climbed during the first two months of the year on the strength of a unified push from the White House and Capitol Hill ahead of the midterm.

Privately, Republicans complain that the president’s sudden shift to tariffs, with threats of trade wars, distracted from the positive impacts of $1.3 trillion in tax cuts and allowed Democrats to regain the upper hand. Concluding that Trump is unreliable, Republicans say it’s their responsibility to turn public opinion around.

“People aren’t talking about it enough, and when people aren’t talking about it enough, that’s a problem,” Ohio Rep. Steve Stivers, chairman of the National Republican Congressional Committee, said Tuesday, of the Tax Cuts and Jobs Act. “Our guys need to be talking about the tax bill more; that’s one of the things that I talked about in conference this morning.”

Republicans in Congress and the Trump administration were on the same page the past few days, promoting the first rewrite of the U.S. tax code to coincide with the annual deadline to pay state and federal income taxes. Vice President Mike Pence has stayed focused on the tax law, campaigning for it during a series of town hall events.

But Trump, the Republican Party’s loudest megaphone, is a mercurial politician. He signed the Tax Cuts and Jobs Act in late December, and spent January and February touting the law, taking every opportunity to credit accelerating economic growth, employee bonuses, new jobs, and wage gains to the overhaul.

That lasted until early March, when the president veered off script to propose steep tariffs on imported steel and aluminum. Trump said the country was suffering from unfair trade practices. Since then, the economy has continued to hum, but approval ratings for the Tax Cuts and Jobs Act have eroded.

[Opinion: Trump's next challenge: Making tax cuts great again]

In a fresh NBC News/Wall Street Journal survey conducted jointly by Democratic and Republican pollsters, the law was underwater: 27 percent approved, 36 percent disapproved. Those results track with private data Republicans have monitored, sparking anxiety about their chances of surviving a tough November election with their House majority intact.

"Republicans have a lot of work in front of them to make sure people understand the benefits of the tax bill, and nobody is going to be driving this but them. They need to understand that it’s not just — we’ve done this, let’s go on to the next thing,” said David Winston, a GOP pollster who advises House and Senate Republicans.

“The signature achievement for Congressional Republicans for this Congress will have been the tax bill — no matter what else they do,” he added.

Yet despite GOP anxiety over the tax laws fade, their spirits are buoyed by what they're seeing on the generic ballot test, gauging which party voters would prefer control of Congress.

The Democrats’ lead on the generic ballot is small enough that if the election were held today, the Republican majority in the House could hold; the party’s advantage in the Senate might even grow. In the polling averages, the Democrats’ generic ballot lead stands at around 6 points.

To be sure, the party would lose seats if they trail the Democrats by 6 points on Election Day. But the GOP isn’t guaranteed to lose the House until they’re locked into a deficit of 7 points or more (some Republican insiders contend that figure should be 8 points or more, perhaps even double-digits.)

In that NBC News/Wall Street Journal poll, conducted April 8 through 11, the Democrats led the generic ballot 47 percent to 40 percent. But in another survey from ABC News and the Washington Post produced during the same timeframe, the Democrats’ lead stood at just 47 percent to 43 percent.

Democrats are dismissing the Republicans’ optimism as false hope. Party operatives say the Democrats polling in individual districts look promising, and they point to the wide lead in voter enthusiasm on the Left across nearly every poll taken this election cycle as a strong indicator of their strength as the fall approaches.

“Democrats have maintained a consistent and significant lead on the generic ballot, and most importantly, district polling shows that Democratic candidates are neck-and neck with or already beating Republican incumbents. On key issues such as affordable healthcare and the tax bill, House Republicans are losing the debate,” said Tyler Law, spokesman for the Democratic Congressional Campaign Committee.