Conservatives in North Carolina have not forgiven Sen. Thom Tillis for initially opposing President Trump’s border emergency, and the Republican could face a competitive primary in 2020 despite ultimately backing the White House.

Tillis voted against a Democratic resolution that would have blocked Trump from declaring a national emergency along the southern border. But grassroots Republicans back home are still fuming about the senator’s original plan to support the legislation on constitutional grounds. Sensing weakness after Tillis essentially flip-flopped, activists and some party officials are talking about fielding a primary challenger.

“The words I heard more times than not were ‘too little, too late,’” Dianne Parnell, the Rockingham County GOP chairwoman, said in a telephone interview with the Washington Examiner. “I feel like he’s just not listening to the people.”

Tillis, 58, supports Trump’s immigration agenda and votes with the president on legislation nearly 95% of the time, according to analysis from FiveThirtyEight. But the Republican base in North Carolina is particularly pro-Trump and views Tillis as an establishment insider. Public opinion polls have shown damage to his personal favorability ratings since late last year.

In November, Tillis received a 34% favorable rating, according to a statewide poll of likely voters conducted by Harper Polling for Civitas Institute, a conservative advocacy group in Raleigh, N.C. In March, the Civitas survey showed Tillis with a 26% favorable rating. Donald Bryson, who runs the Civitas Institute, attributed the drop in Tillis' numbers to "a grassroots problem within the GOP base."

What happened?

Tillis, in a splashy Washington Post op-ed published in February, vowed to oppose Trump’s plan to call a national emergency. The president took that approach to claim the power to spend federal money on border security that Congress had declined to authorize. The senator explained that his opposition was about preserving the separation of powers.

Then, after heavy lobbying by the White House and under pressure from grassroots Republicans, including threats of a primary challenge, Tillis backtracked. But the move has done little to appease pro-Trump activists, with some party insiders saying Tillis would have emerged stronger had he maintained his opposition to the border emergency and weathered the storm from the grassroots.

“He did himself some real harm. It’s one of the classic political mistakes,” said Carter Wrenn, a veteran Republican operative in North Carolina who is advising Garland Tucker, a potential Tillis primary opponent. “You flip-flop and that’s usually not a good move.”

Tillis pointedly rejects the flip-flop label.

[Opinion: Trump declaring a national emergency to build border wall with McConnell's support will come back to haunt conservatives]

In an interview Wednesday, the senator credited his change of heart to discussions with the White House about reducing presidential powers to call emergencies. Trump, Tillis said, signaled a willingness to back changes in existing law, assuaging his concerns about constitutional overreach. The senator shrugged off complaints from the Republican base, suggesting there is a contingent of activists who would never be satisfied.

“Part of that base is a always a base that’s been upset with me back when I was speaker and trying to manage a complex agenda,” said Tillis, who was speaker of the House in the North Carolina Legislature before he was elected to the Senate in 2014.

Pat Smith, president of the North Carolina Federation of Republican Women, is a big Tillis supporter, and she predicted he would avoid meaningful opposition in the primary. “I think he’s very strong. I like him,” she said.

But for the time being, at least, Tillis stirred up a hornet’s nest of intra-party opposition. Rep. Mark Walker, a conservative favorite, has indicated he might run against Tillis in the GOP primary. Tucker, a wealthy former CEO of a North Carolina investment firm, also is examining a primary challenge. He has the money to self-fund a campaign. Others also might also consider a run.

Tillis supporters have some grounds for reassurance.

The threshold for winning a primary is just 30 percentof the vote. If Tillis runs against multiple challengers, he would have a built-in advantage. And some believe Walker and Tucker may not be strong enough candidates to overcome a sitting senator.

Correction and Clarification: After a recent change in the law, the threshold for winning primaries in North Carolina without a runoff is now 30 percent of the vote, not 40 percent, as previously reporterd. Also, Republican consultant Carter Wrenn should have been identified as an advisor to Garland Tucker.