President Trump's sagging public support is not only threatening his agenda in Congress, but it may also risk steep GOP losses in 2018.

Trump's job approval rating is mired in the low 40s, historically low for a president short of the 100-day mark. That has senior Republicans worried that those numbers will sabotage efforts to repeal Obamacare and reform the tax code, and that, in turn, could leave the GOP with no accomplishments to talk about in the next election.

"At some point, if the White House can't put wins on the board, you're going to see members of Congress go it alone. Republican voters will begin to weigh on their congressman to get things done — with or without President Trump," said a senior Republican House aide, who requested anonymity in order to speak candidly. "Congress likes working with a popular president. They don't like defending an unpopular one time after time."

"It's important to have accomplishments," added Sen. Rob Portman, R-Ohio, who serves on the Finance Committee, a key panel with jurisdiction over health care and taxes.

Trump's national job approval rating was 41.4 percent in the RealClearPolitics average; 53 percent disapproved of him. Gallup's daily Trump track pegged the president at a 38 percent approval rating.

In his home state of New York, Trump is struggling in regions where Republicans usually perform well.

A Siena College poll conducted March 19-23 showed his personal favorable numbers at 38 percent in the suburbs and upstate, where his support in the past has been strong. The president's job approval also was low, as 50 percent of upstate voters and 53 percent of suburban voters gave him a "poor" rating.

That doesn't bode well for building Republican coalitions for his agenda, especially if Trump continues to spar with the Freedom Caucus, as he did Thursday over Twitter. The president's chances of garnering Democratic votes for his priorities remain slim.

According to a tracking poll conducted for Priorities USA, a Democratic-aligned super PAC, the percentage of Trump voters who said their opinion of the president had increased based on "what they have heard about him recently" dropped from 80 percent to 49 percent.

That erosion occurred from early to late March, during roughly the same period as the debate over the American Health Care Act, the GOP package to replace former President Barack Obama's healthcare law.

Republican operatives and lawmakers admitted they are concerned about Trump's polling trend but say he is still in good shape with Republicans.

Brad Todd, a Republican strategist, referred to Trump's national numbers as "deceptively irrelevant" because they are being driven by unusually high disapproval in the Democratic strongholds that are adjacent to Interstates 5 and 95 and dominate the west and east coasts.

Republican insiders say their data show that the president is still strong in GOP districts and states, putting him in a position to get things done in Congress.

"He's still in a honeymoon and people are still hoping to get big things done," said Sen. Bob Corker, R-Tenn. "The president will sign big things, so I don't think the dynamic's changed all that much."

Most Senate seats contested in 2018 are in Republican-leaning states but are held by Democrats. For the moment, that is expected to cushion the GOP's 52-48 majority. The environment could be more challenging for House Republicans, who hold a 24-seat majority and are defending 23 districts Hillary Clinton won in November.

With control of the White House, House and Senate, that puts pressure on Republicans to deliver legislative accomplishments.

"Eventually, the president will be accountable to Republican voters for putting points on the scoreboard," Todd said. "That's why it is imperative that he roll up his sleeves and get the Obamacare repeal done, whether it bores him or not."