Once the exception, filibusters are now the norm in the Senate. But it is exceedingly rare for senators to filibuster a signature initiative by a president of their own party.
That's what the Democratic minority did Tuesday by blocking a motion to debate Trade Promotion Authority, a vehicle to fast-track a trade pact that President Barack Obama is negotiating and allow Congress to have the final word on it.
While every Republican was on board, the only Democrat in the 46-member caucus who voted yes to open the debate was Tom Carper of Delaware. The 52-45 vote fell short of the 60 needed to advance.
Congressional scholars said the last time a party filibustered its own president's signature initiative occurred in 2007, when Republican senators led opposition to President George W. Bush's push to overhaul immigration law.
That vote in 2007 effectively stopped Bush's immigration plan. The trade deal, though, could live to see another day, as Senate Majority Leader Mitch McConnell signaled after the failed vote that he was willing to find another way forward.
The most prominent filibusters against presidents of one's own party occurred in the first half of the 20th century, congressional scholars said.
"There's a half-century of defeats of civil rights bills (anti-lynching, fair employment, poll tax and housing bills)—at the hands of Southern Democrats with Democratic presidents in the White House," Sarah Binder, a political scientist at George Washington University, said in an e-mail. "Can't come up with examples post immigration reform that neatly fit the bill."