Will tax cuts save Republicans in 2018?

But if tax cuts are a winning message, the evidence so far is mixed. And Democrats are determined to turn Republicans' signature legislative achievement against them, framing it as a giveaway to the wealthy that will threaten Medicare and Social Security.

At a meeting last week following Republicans' defeat in a Pennsylvania special election, House Speaker Paul Ryan emphasized the importance of campaigning on the tax law. Rep. Steve Stivers, head of the National Republican Congressional Committee, told members that despite the difficult political environment, it was possible to hold the majority, especially if the GOP effectively sells the tax overhaul, the party's singular legislative achievement.

Corry Bliss, who heads two major GOP groups -- the nonprofit, issue-oriented American Action Network and the political action committee Congressional Leadership Fund -- put it this way last week: "Answer this question and I will tell you if we keep the House or not. In 10 months, does the middle class think we cut their taxes? Every member of the Republican Party should be spending all of their time selling the tax plan. Everything else is a waste of time and money."

But in Pennsylvania's 18th District -- where Democrats swung the race 20 points from 2016 -- Republicans moved away from tax cuts in the final days of the campaign and focused instead on issues like crime and sanctuary cities -- a sign that taxes were not the effective message that party leaders hoped it would be.

The Republican there, Rick Saccone, embraced the tax cuts during the campaign, saying in a debate that they're "not the 'crumbs' that Nancy Pelosi and her crew on the left say," and that "people are very happy to have the bonuses that they received."

Democrat Conor Lamb campaigned against them, calling the overhaul a "giveaway" to the wealthy and adding, "we didn't need to add a penny to our debt to have the tax cut for our working and middle-class people."

A series of factors contributed to the surprise outcome in Pennsylvania, including Saccone's much-criticized campaign and listless fundraising. Still, it was an ominous test case, and it's not the only warning sign.

Nationally, polls show Republicans continue to face a 9- to 10-point deficit on the generic ballot, despite passage of the tax plan in December.

And recent polling on the tax plan suggests that public support is split at best and underwater at worst. A Quinnipiac poll released last week found that 36% of American voters supported it, compared to 50% who opposed it. That's an improvement from November, when Quinnipiac first started polling on the tax plan and found support at just 25%. But support appears to be leveling off after a high-water mark of 39% in early February.

A recent Monmouth University poll found opinion on the tax cuts split, with 41% of respondents approving and 42% disapproving.

In Indiana, where vulnerable red-state Democrat Joe Donnelly is running for reelection to the US Senate, both Republicans and Democrats see the tax cuts as an attack line.

Last week, Americans for Prosperity -- one of the conservative Koch brothers political spending vehicles -- announced a second wave of ads worth $4 million targeting Donnelly and and a fellow red-state Democrat, Sen. Claire McCaskill of Missouri, for their opposition to the tax cuts.

In the ad, a small businesswoman says: "Because of the tax cuts, I'm able to invest more money back into my business. When I heard that Joe Donnelly voted against the tax cuts, I was very disappointed. Senator Donnelly just doesn't realize the impact that these tax cuts have on the everyday person and the everyday business. Joe Donnelly let Hoosiers down."

On the other side, the Senate Majority PAC, a leading Democratic super PAC, released an ad backed by $500,000 that defends Donnelly for opposing the plan.

In the spot, a man loads his truck, drives to work at a manufacturing plant and says, "They say the new tax law was written to help families like mine, but so much of it goes to millionaires and big corporations. And it's adding trillions to our national debt. Now they're talking about cutting Medicare and Social Security to pay for it. Our Senator, Joe Donnelly, voted no. He supported a middle class tax cut. Out of state billionaires are attacking him for it. But I know Joe Donnelly is always looking out for Indiana."

Chris Hayden, communications director for the group, told CNN, "When you tell voters that this adds $1.8 trillion to the deficit, that [Republicans] want to attack Social Security and Medicare to pay for it, that health care premiums are going up even though health care companies got billions from the tax bill -- opinions swing back in favor" of Democrats. "We believe it's a winnable issue."

And House Majority PAC, another leading Democratic super PAC focused on the lower chamber, also made a small TV ad buy last week attacking GOP Speaker Paul Ryan and highlighting "the Wall Street millionaires that Ryan gave billions in tax breaks."

Republicans argue that the tax overhaul will take time to sell to a wary electorate after its rushed passage. And some party members are already hinting at a so-called "phase two" of tax legislation, though the details are still unclear.

Kevin Brady, chairman of the House Ways and Means Committee, told Fox News recently, "We think even more can be done," and President Donald Trump has expressed support for the idea, telling a business executive roundtable, "It's going to be something very special, Kevin Brady's working on it with me."

Matt Gorman, communications director for the NRCC, told CNN that tax reform is the "hub of a wheel" of economic policy wins, "one of if not the biggest legislative accomplishment" of the Republican Congress. He touted the cuts as part of a "broader agenda that speaks to people's pocketbooks," including efforts on vocational training, student loans, and childcare. Gorman also noted that "one of the best ways to get tax cut numbers up is keep reading local press," and cited recent headlines about wage increases, bonuses and hiring as a result of the tax legislation.

The polling is "on the right side up and rising," he said.