WASHINGTON -- Sen. Cory Booker (D., N.J.) announced Wednesday that he will vote against President-elect Donald Trump's nominee for secretary of state, Rex Tillerson, backing up his previous criticism.

UPDATED: Later in the day, Sen. Bob Casey (D., Pa.) announced his opposition to a trio of Trump cabinet picks: Jeff Sessions, nominated for attorney general, Betsy DeVos, nominated for education secretary, and Scott Pruitt, tapped as environmental secretary.

The opposition is not surprising. Booker and just about every Democrat have been skeptical of Tillerson ever since the former ExxonMobil CEO was nominated. And Booker, widely seen as a potential presidential candidate in 2020, has been eager to take high-profile stands against Trump nominees, breaking Senate tradition to testify last week against a fellow Senator, Sessions.

Booker, a member of the Senate Foreign Relations Committee that grilled Tillerson last week, said his opposition was assured once he saw Trump's comments over the weekend calling NATO "obsolete."

"Recent statements from President-elect Trump describing NATO as 'obsolete' and his attacks on the politics of our closest allies further cements my view that America needs a Secretary of State who will project confidence – not further insecurity – about our motives and values," said a Booker statement Wednesday morning. "America is a beacon of democracy and hope to the world. I do not believe that Mr. Tillerson is the right choice to ensure that this legacy continues. ‎For these and other reasons, I will oppose his confirmation."

While Democrats do not have the votes to block Tillerson, the nominee has also drawn sharp scrutiny from a handful of Republicans over his and Trump's seeming affinity for Russia and its president, Vladimir Putin, making his confirmation one of the most tenuous. The GOP has 52 votes in the Senate, so they can afford few defections.

The Philadelphia region's only Republican senator, Pennsylvania's Pat Toomey, has not said how he will vote on Tillerson.

Democrats from the area have expressed deep skepticism without definitively saying they will oppose the nominee. Two, Sens. Robert Menendez (D., N.J.) and Chris Coons (D., Del.) also sit on the foreign relations committee, which is expected to vote on the nomination Monday.

It's open question if Tillerson will win a majority of votes on the committee, where the GOP has a one-vote edge and Sen. Marco Rubio (R., Fla.) has raised stern questions. But the panel's chairman, Sen. Bob Corker (R., Tenn.) told CNN this week he would move the nomination to the Senate floor even if Tillerson fails to win the committee's support.

It's rare for a cabinet nominee to get blocked. The last time the Senate voted down a president's pick was in 1989, under President George H.W. Bush. Six nominees since then have withdrawn before votes were held.

Casey sits on the health and education committee, which vetted DeVos this week. He cited her refusal, in response to his question, to commit to defending campus sexual assault laws contained in Title IX. "Ms. DeVos would take public education in our nation backward and make it harder for public school students in Pennsylvania to achieve their potential," Casey said in a statement.

He criticized Sessions over the nominee's views on voting rights laws, and said he worried that Pruitt, the attorney general of Oklahoma, would weaken critical environmental protections.

You can follow Tamari on Twitter or email him at jtamari@phillynews.com.