WASHINGTON — Last week, when Democrats in the U.S. Senate mobilized against President Donald Trump’s emergency declaration at the border — they found themselves with an unusual ally.

Republican Sen. Pat Toomey of Pennsylvania, who sided with Trump almost 90 percent of the time in the last Congress and 80 percent so far this year, was among the 12 members of the Senate GOP who crossed the aisle to vote with Democrats to terminate the emergency declaration.

For Toomey’s critics back home in Pennsylvania — who have spent the past two years since Trump’s election deriding the Lehigh Valley native as a Trump apologist, lambasting him for what they see as a lack of public accessibility, and picketing his Pennsylvania offices — the very public break with the White House was a rare and surprising departure.

But political observers say the break with Trump is in character for a conservative Republican who hasn’t been afraid of bucking his party when he sees the need.

“When he’s been asked about his vote to terminate the national emergency, he’s provided his explanation (as he laid out in the Philadelphia Inquirer op-ed) and constituents are generally satisfied with his answer,” spokesperson Bill Jaffee told the Capital-Star.

Toomey, by implication, found himself the target of the White House’s ire, as Trump took to Twitter to hammer Democrats who stymied one of his tentpole campaign promises — a wall at the U.S.-Mexico border.

“I look forward to VETOING the just passed Democrat inspired Resolution which would OPEN BORDERS while increasing Crime, Drugs, and Trafficking in our Country,” Trump tweeted. “I thank all of the Strong Republicans who voted to support Border Security and our desperately needed WALL!”

Toomey defended the vote on NBC’s “Meet the Press with Chuck Todd,” saying it was all about the separation of powers and not the wall, which he favors and thinks should be funded.

“For decades now, Congress has been transferring way too much constitutional authority from the legislative branch to the executive,” he said. “That’s very bad for a representative democracy, for a republic such as ours.”

Toomey pledged to continue the fight even if Trump’s veto holds. He is pushing for legislation that would that would require presidents to obtain congressional approval to extend emergency declarations beyond 30 days. Currently, Congress can end an emergency declaration only by passing a resolution that can withstand a presidential veto.

“I’m hoping we can restore some of this authority to Congress, where it belongs,” Toomey said — a message he is continuing to spread in the state this week while Congress is in recess.

‘Threading the needle’

In his vocal and visible challenges to Trump, Toomey is differentiating himself from other members of his party. “Well-behaved Republicans don’t do that,” said Kristin Kanthak, a political science professor at the University of Pittsburgh.

But because he opposes Trump on process, not policy — at least on this issue — Toomey can successfully “thread this needle,” said Dana Brown, executive director of the Pennsylvania Center for Women & Politics at Chatham University. Politically, it’s a “safe” vote, in part, because he’s not up for reelection until 2022 and won’t have to react to Republican fervor over his stance for a few years, she said.

The vote also aligns with his unique identity as a conservative ideologue. “In a lot of ways, this is really typical of who Pat Toomey has been his whole career,” Kanthak said.

After serving three terms in the House, Toomey mounted a primary challenge to then-GOP Sen. Arlen Specter in 2004. He lost and went on to serve as president of the Club for Growth, an anti-tax group that targets fiscally moderate lawmakers, including Republicans.

Toomey pledged to take on Specter again in the 2010 GOP primary, prompting the veteran senator to switch parties and run as a Democrat. Specter failed to fend off a primary challenge from the left, and Toomey won the seat in 2010 and again in 2016.

An ‘oddity’ in the Senate

Because he did not rise up through the GOP ranks, Toomey is freer to challenge Trump than other members of his party, making him an “oddity” in a hyper-partisan era.

“Republicans are supposed to go along with the Republican president,” Kanthak said. “It doesn’t matter what he says, you do it because he’s a Republican. Toomey doesn’t play that way.”

At the same time, he hasn’t hesitated to challenge Trump when he crosses conservative ideology, Kanthak said, citing his recent “broadside” against Trump’s efforts to use national security threats to justify new tariffs on steel and aluminum.

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“On trade,” Toomey said in a statement, “I continue to believe the administration is taking us down the wrong path.”

Late last year, Toomey voted against Trump-signed legislation that granted billions in aid to U.S. farmers and retained existing food stamp provisions, calling it “a wasted opportunity to rein in excessive spending and end corporate welfare.” On the flip side, he continues to work with Democrats — and against many in his party — to enact gun safety legislation.

In general, Toomey has been able to keep his differences with the GOP “under the radar,” Kanhtak said. But he’s drawing more attention as conflicts sharpen over core conservative issues.

That’s a stance that carries some political risk in a state that handed Democrats major victories in last fall’s midterms, Kanthak said.

“He’s in a really tough spot,” she said.