AMNA NAWAZ: President Trump's pick to be the chairman of the Joint Chiefs of Staff, the president's most senior military adviser, told senators during his confirmation hearing today that he would give the president candid guidance and stay out of politics.

GEOFF BENNETT: But he also said he was willing to push back if ever asked to do something unconstitutional.

And he signaled concern about recent messages sent by senior national security leaders over an unclassified messaging app.

Here's Nick Schifrin.

MAN: Committee will come to order.

NICK SCHIFRIN: Today, in front of the Senate Armed Services Committee President Trump's chairman of the Joint Chiefs nominee promised to keep himself and the military apolitical.

SEN. MARK KELLY (D-AZ): Are you willing to get fired from this job for doing the right thing and following the Constitution?

LT. GEN. DAN CAINE (RET.

), Chairman of the Joint Chiefs of Staff Nominee: I am.

NICK SCHIFRIN: But, immediately, retired Lieutenant General Dan Caine, nicknamed Razin', was dragged into a partisan fight.

Democratic senators raised messages sent by Secretary of Defense Pete Hegseth on the unclassified app Signal the two weeks ago revealed the exact timing in weapon systems before U.S. strikes on Yemen.

LT. GEN. DAN CAINE (RET): We should always preserve the element of surprise.

SEN. MAZIE HIRONO (D-HI): Is discussing the kinds of matters that were discussed on Signal preserving the element of surprise?

LT. GEN. DAN CAINE (RET): Given the fact that the chairman and ranking member have asked for an investigation, I don't want to comment on the particulars.

SEN. MAZIE HIRONO: I would say that, if you're truly speaking truth to power, that the answer would obviously be, no, it does not preserve the element of surprise.

SEN. KIRSTEN GILLIBRAND (D-NY): If you see classified information or war plans information or tactical information an unclassified chain, what would you do to stop the conversation?

LT. GEN. DAN CAINE (RET): Well, I think I would weigh in and stop it if I was a part of it, but in this case I wasn't.

NICK SCHIFRIN: Caine, like his father, was a decorated fighter pilot, whose last position was the senior military liaison to the CIA.

He spent much of his career in special operations forces, including in the counter-ISIS campaign in Iraq, where in 2018 he briefed President Trump, as the president recounted last year.

DONALD TRUMP, President of the United States: So I walked down and this is where I met general Razin' Caine.

And what's your name?

General, what's your name?

And he gave me his name.

What's your name?

Sir.

Yes, sir.

And I love you, sir.

I think you're great, sir.

I will kill for you, sir.

DONALD TRUMP: Then he puts on a "Make America Great Again" hat.

You're not allowed to do that, but they did it.

SEN. ROGER WICKER (R-MS): General Caine, did you wear a MAGA hat in front of the president?

LT. GEN. DAN CAINE (RET): No, sir.

SEN. ROGER WICKER: Did you wear a MAGA hat at any time?

LT. GEN. DAN CAINE (RET): No, sir.

I think - - I went back and listened to those tapes, and I think the president was actually talking about somebody else, and I have never worn any political merchandise or said anything to that effect.

SEN. MAZIE HIRONO: If you did wear a MAGA hat, would that constitute partisan political activity by a uniformed officer?

LT. GEN. DAN CAINE (RET): I think it probably would, yes, senator.

NICK SCHIFRIN: Caine's two predecessors have both run a fowl of President Trump.

Air Force General C.Q.

Brown, chosen by President Biden, was fired.

And General Mark Milley, chosen by President Trump, but later deeply critical of him, now faces post-retirement investigations and his portraits at the Pentagon have been removed.

SEN. MAZIE HIRONO: General Brown's dismissal reveals, once again, President Trump's intention to install yes-men and women with fealty to him and not to the Constitution or the American people.

NICK SCHIFRIN: But Republican senators praised Caine's experience, which does not include commands of a military service or regional headquarters.

That will require Caine to receive presidential exemptions.

SEN. KEVIN CRAMER (R-ND): While I agree that the dismissal of your predecessor was unfortunate, it's behind us.

And I think -- I cannot think of a better person to replace General Brown than you.

SEN. JONI ERNST (R-IA): We are in very challenging times.

We all recognize this.

We need a change.

You are that change.

NICK SCHIFRIN: And even some Democrats said at the end of the day, they wanted Caine to give the president his best military advice.

SEN. ELISSA SLOTKIN (D-MI): Will you push to be in that room?

LT. GEN. DAN CAINE (RET): Yes, ma'am.

SEN. ELISSA SLOTKIN: Good.

We want you there.

NICK SCHIFRIN: For the "PBS News Hour," I'm Nick Schifrin.