The White House is watching him. Republicans in Washington and battleground North Carolina are taking note. And Sen. Thom Tillis is again in the tense middle of a test of how closely he'll stand with President Donald Trump.

"Just another day at the office," Tillis, R-N.C., told reporters on the way into yet another Republican meeting on whether to reverse the president's declaration of a national emergency to pay for his border wall. Just over an hour later, the GOP senators emerged with an answer: The vote is going to go against Trump, possibly with Tillis' help.

Tillis' strong statements of opposition to Trump's use of executive power are not the first time the former IBM consultant and state House speaker has defied the president, who often views other Republicans in terms of their loyalty to him. Tillis partnered with Democrats earlier this year on legislation to protect special counsel Robert Mueller's job investigating the president's campaign and Russia's interference in the 2016 election.

Tillis is the latest Republican to signal to the "I alone can fix it" president that Trump's power is not absolute, even in GOP circles. Increasingly, Republicans have been willing to defy Trump publicly on matters ranging from the murder of journalist Jamal Khashoggi to the president's apparently supportive stance toward Russia.

For Tillis, 58, decisions on how and when to support or oppose Trump are clearly shaded by his expected bid for a second Senate term next year — and the drive to hold off any strong challengers. In a floor speech and an op-ed in The Washington Post, Tillis made clear that he shares Trump's concerns about border security. But he firmly declared that Trump's effort to go around Congress to pay for his wall overstepped the Constitution's separation of powers.

"These are the reasons I would vote in favor of the resolution disapproving of the president's national-emergency declaration," he wrote.

Doing so would plant Tillis firmly among the Senate's influential centrists, such as Sen. Susan Collins, R-Maine.

Only Tillis' vote this week was not so firm at times. In meetings with Senate Republicans and Vice President Mike Pence, Tillis suggested that he might vote Trump's way — but only if the president agrees to limit a president's power to get federal money by declaring a national emergency. At one point Wednesday, one observer estimated as many as 15 Republican senators would vote against the president.

The uncertainty, and Tillis' apparent wavering, seemed aimed at appeasing the GOP base back home as well as independents.

"He needs both an enthusiastic party base as well as at least some unaffiliated voters to win," said Eric Heberlig, a political science professor at the University of North Carolina at Charlotte. "To appeal to one is basically to shut out support for the other."

While Tillis has been a reliable conservative vote during his first term, he's also pressed for bipartisanship, hardly letting a few days go by this year without news releases highlighting bills he's introduced with Democrats.

Republicans back home had questioned Tillis' support for Trump when he co-sponsored the legislation to protect Mueller's job. Tillis said in January he didn't believe Trump would fire the special prosecutor but called that bipartisan bill "good government policy with enduring value across the current and future administrations."

Dianne Parnell, chairwoman of the Rockingham County Republican Party, said Wednesday she's weighing whether Tillis' apparent second thoughts on the border emergency resolution changes her view of him.

"We want him to support our president," Parnell said, adding that now is not the time for Tillis to reach across the aisle and blaming Democrats for hyper partisanship. "I would be delighted if he changed his mind."

Suspicion about Tillis by Republican activists has been around for years. While Tillis helped lead the charge to conservative GOP control in the state legislature for the first time in 140 years in 2011, some on the far right didn't believe he was conservative enough.

That required Tillis to take on credible tea party adversaries in the 2014 Republican Senate primary before upsetting Democratic Sen. Kay Hagan later that year in a race where the two sides spent more than $100 million.

State Democrats are jumping on his potential change of heart on the resolution, with party spokesman Robert Howard saying Wednesday that it didn't take long for Tillis "to lose his spine and fall right back in line with President Trump."

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Robertson reported from Raleigh, North Carolina. Associated Press writer Lisa Mascaro and researcher Rhonda Shafner contributed to this report.