GEOFF BENNETT: Tomorrow marks one year since Donald Trump reentered the White House, and all this week we will be delving into various aspects of his presidency at this moment.

AMNA NAWAZ: Tonight, congressional correspondent Lisa Desjardins looks at his expansion of presidential power and how it's testing Congress and the system of checks and balances rooted in the Constitution.

LISA DESJARDINS: The second Trump White House is a blitz of executive actions... DONALD TRUMP, President of the United States: What we're going to be doing is a 25 percent tariff on all cars that are not made in the United States.

We're terminating those programs, and they're going to be terminated on a permanent basis.

LISA DESJARDINS: ... and use of the military and federal authorities in American cities and abroad.

DONALD TRUMP: All political and military figures in Venezuela should understand what happened to Maduro can happen to them.

LISA DESJARDINS: A forceful display of power that often leaves Congress sidelined... SEN.

LISA MURKOWSKI (R-AK): I think it's important to send the message.

LISA DESJARDINS: ... and trying to weigh in, like last week, when these senators met with a minister from Greenland, whose country Trump wants to take over, I asked about their power.

SEN.

LISA MURKOWSKI: Every day, we should be thinking if we are fully asserting our authorities under the Constitution.

LISA DESJARDINS: Is Congress right now?

SEN.

ANGUS KING (I-ME): No.

SEN.

LISA MURKOWSKI (R-AK): Some days, no.

SEN.

ANGUS KING: Congress has abdicated its power largely.

I'd say it's the seventh inning.

We're behind 4-3.

But the game isn't over.

SARAH BINDER, George Washington University: The big, long picture here over the 20th century into these years of Trump is a loss of congressional power.

LISA DESJARDINS: Sarah Binder is a professor at George Washington University, focused on Congress.

She points to how Congress grew and crises like the Great Depression and World Wars leading to Congress giving more power to past presidents.

But now there's another factor.

SARAH BINDER: We're in a world of high, intense partisanship, polarization, which means that the loyalties of members of Congress are quite often more to the president of their party than to their responsibilities as lawmakers.

LISA DESJARDINS: Add to a Congress driven by party loyalty, a president who uses social media to threaten those in Congress who oppose him.

SARAH BINDER: This president thinks he has no need for Congress.

LISA DESJARDINS: The Constitution starts with Congress.

It is Article I, and it is twice as long as article II, which covers the president.

Democratic lawmakers especially point this out.

REP.

JAMES WALKINSHAW (D-VA): I actually take the view that Congress has Article I powers and the founders intended Congress to be the more powerful branch of government.

LISA DESJARDINS: We spoke with 71 current members of Congress for this story.

The vast majority, members of both parties, bemoaned what they see as a long-term loss of power for Congress.

Some see it now as at historic lows.

But in our hallway conversations, we heard resignation.

Few members felt responsible for or talked of tackling the problem.

We asked Republican Ryan Zinke, who served as Cabinet secretary in Trump's first term and now is a House member for Montana.

REP.

RYAN ZINKE (R-MT): Congress over the years has let loose a lot of the power, primarily power of the purse.

LISA DESJARDINS: But next we asked him about Trump specifically, how he has sent the National Guard to states and cities uninvited, renamed the congressionally established Kennedy Center after himself, and ripped up congressional spending plans, dismantling agencies like USAID.

Zinke then defended the president.

REP.

RYAN ZINKE: Fraud, waste, and abuse is never congressional intent.

And the president is within an authority to remove and stop funding of fraud, waste, and abuse.

So... LISA DESJARDINS: But he has - - he removed entire agencies, USAID, that were established by Congress to be independent.

REP.

RYAN ZINKE: Well, but also was it in the best interest of the United States and did it follow and support U.S.

policy?

LISA DESJARDINS: But my question is, Congress hasn't weighed in on that at all.

REP.

RYAN ZINKE: Well, I would say the president is within his constitutional right to make sure we don't have fraud, waste and abuse and taxpayer dollars are spent wisely.

LISA DESJARDINS: That is broad leeway, which Trump also is getting in Venezuela, with Republican senators last week providing just enough votes to bat down an attempt to limit his powers there.

J.D.

VANCE, Vice President of the United States: And the point of order is sustained.

LISA DESJARDINS: Which brings us back to Greenland and Denmark, where protests about the U.S.

are rising and where senators visited this weekend.

But it was mostly Democrats.

And it's not clear what they will do about the president's threats.

SEN.

CHRIS COONS (D-DE): One of the things that's been made clear to us is that the people of Denmark are very anxious, are quite concerned, and the people of Greenland even more so.

LISA DESJARDINS: Hovering, what seems a basic question, doesn't Trump need congressional approval to act to send a military force to Greenland?

I asked House Speaker Mike Johnson this earlier this month.

He said, maybe, but maybe not.

REP.

MIKE JOHNSON (R-LA): Under Article II, as we talked about in the room, the president has broad authority as a commander in chief... LISA DESJARDINS: But why Greenland?

REP.

MIKE JOHNSON: ... as all previous presidents have.

LISA DESJARDINS: Democratic Illinois Senator Tammy Duckworth, a combat veteran, responded.

SEN.

TAMMY DUCKWORTH (D-IL): I strongly disagree with him.

That is absolutely not what the Constitution says.

But, in fact, it is where Republicans are.

They have basically given their powers over to the White House.

JAMEEL JAFFER, George Mason University: President Trump has been running the table on the legislative branch for two terms now in a row.

LISA DESJARDINS: Jameel Jaffer is a law and national security professor at George Mason University and a Republican.

He sees Trump as within the norm so far in terms of military action overseas, but he sees unique imbalance in power otherwise.

JAMEEL JAFFER: These are really interesting and perhaps dangerous times, an overly assertive executive without any constraint by Congress, with limited constraints by the court.

We have seen how that operates in other countries.

We haven't seen that here historically in America.

LISA DESJARDINS: Republicans are divided on the arc of power.

REP.

JOHN RUTHERFORD (R-FL): Look, I think the balance is where it's always been.

We have the appropriations process that is a check and balance.

SEN.

RON JOHNSON (R-WI): In general, in terms of our balance of powers, it's not very balanced anymore, mainly because Congress has willingly over the decades given up constitutional authority.

LISA DESJARDINS: While Democrats see them as throwing away power right now.

REP.

ALEXANDRIA OCASIO-CORTEZ (D-NY): As it pertains to President Trump, it is unprecedented.

There is virtually no pushback whatsoever.

LISA DESJARDINS: Congress overall recognizes, but has yet to reckon with the situation.

The first branch in the Constitution is often a second thought in critical decisions for the country and the world.

For the "PBS News Hour" I'm Lisa Desjardins.