JUDY WOODRUFF: The battle over repealing Obamacare has been joined at the U.S. Capitol tonight. Both the sitting president and the incoming vice president were there, making their cases for reprieve or repeal.

Lisa Desjardins has our report.

LISA DESJARDINS: At the Capitol, President Obama and vice president-elect Pence were minutes apart in arrival, and light years apart in purpose. Mr. Obama, in a private meeting with Democrats, urged them to defend his signature health care law.

PRESIDENT BARACK OBAMA: Hello, everybody. Happy new year.

LISA DESJARDINS: While a few floors up, Mr. Pence rallied his former Republican colleagues in the House to dismantle it. He said afterward that Republicans are paying attention to how to avoid ripple effects of repeal, for individuals and the market.

MIKE PENCE, Vice President-Elect: We're working right now, the White House staff is, on a series of executive orders that will enable that orderly transition to take place, even as the Congress appropriately debates alternatives to, and replacement of, Obamacare.

LISA DESJARDINS: Pence gave no specifics, saying it's too soon.

Those who will have to wrestle with the specifics, rank-and-file Republicans, focused on their energy.

MAN: It more a pep rally.

LISA DESJARDINS: As for a timeline on a Republican replacement plan, Trump transition team member Chris Collins and others offered this.

REP. CHRIS COLLINS (R-N.Y.): We're going to have to, over the next six months, put that pen to paper.

REP. KRISTI NOEM (R-S.D.): Oh, absolutely. That's our agreed-upon agenda, is to get it done within six months. We're not wholly united on one idea right now, but I would say we're definitely in a better spot than we were six months ago.

LISA DESJARDINS: This is a good time to explain how Republicans plan this repeal. Step one, what's happening now, is a procedural move, not the actual repeal yet. Both chambers will instruct committees to submit budget ideas by January 27.

Then, step two, inside those budget resolutions will be the repeal of all or part of Obamacare. Why budget resolutions? They require just 51 votes in the Senate. So those budget plans will be the actual repeal. Republicans hope to do that by March.

Step three is the replacement, and that could come in several pieces. As you heard, many Republicans want to propose something within six months. All of those decisions, affecting nearly every American, are politically precarious. President-elect Trump himself warned Republicans on Twitter today to be careful.

President Obama didn't speak publicly at the Capitol today. Instead, Democratic leaders did in a fiery news conference, insisting Republicans could make things worse, a point highlighted in a new Trump-inspired motto.

SEN. CHUCK SCHUMER, Minority Leader: The Republican plan to cut health care wouldn't make America great again. It would make America sick again, and lead to chaos, instead of affordable care.

LISA DESJARDINS: Democrats are, of course, in the minority in both chambers, but they were joined by one Republican this afternoon. Senator Rand Paul of Kentucky voted against the first procedural move toward health care repeal, citing cost concerns. It passed without him.

And even if Rand Paul continues to be a no vote, Republicans have enough votes to repeal Obamacare. What is not clear is if they have enough votes for any one replacement plan. There will be a lot of maneuvering and speeches over the next couple of weeks, but, Alison, what is key to watch for is the decision Republicans make as to when the Obamacare repeal should take effect, quickly or over years — Alison.