Mr. Obama has effectively reversed his position and now said he believes that such actions can be “legally unassailable,” as a senior White House official put it last week. Mr. Obama is expected to announce plans soon to expand the program for Dreamers to shield up to five million people from deportation and provide work permits for many of them.

The president insisted over the weekend that he had not changed his position. During a news conference in Australia, the president said that his earlier answers about the limits of his executive authority were prompted by people who asked him whether he could enact, by fiat, a bipartisan immigration bill that had passed the Senate, which would have provided a path to legalization for more of the 11 million unauthorized people in the United States.

“Getting a comprehensive deal of the sort that is in the Senate legislation, for example, does extend beyond my legal authorities,” Mr. Obama said Sunday. “There are certain things I cannot do.”

In fact, most of the questions that were posed to the president over the past several years were about the very thing that he is expected to announce within a matter of days: whether he could do something to reduce deportations and keep families together if Congress would not act.

The president was pressed on that very issue during a Google Hangout in February 2013. An activist asked whether he could do more to keep families from being “broken apart” while Congress remained gridlocked on immigration legislation.

“This is something that I have struggled with throughout my presidency,” Mr. Obama said. “The problem is, is that I’m the president of the United States, I’m not the emperor of the United States. My job is to execute laws that are passed.”

Officials have said the president could announce a series of executive actions as early as this week. The move comes after a concerted lobbying campaign by immigration advocates demanding presidential action in the face of 400,000 deportations every year. And it reflects the president’s mounting frustration that Republicans have blocked all efforts to pass immigration legislation.

At the news conference in Australia over the weekend, Mr. Obama implored Congress to pass a bill that would secure the border, revamp the legal immigration system and legalize many of the 11 million unauthorized immigrants living in the United States.

“Give me a bill that addresses those issues,” Mr. Obama said at the conclusion of the G-20 summit in Brisbane, Australia. “I’ll be the first one to sign it and, metaphorically, I’ll crumple up whatever executive actions that we take and we’ll toss them in the wastebasket.”

White House officials said they did not believe that Republicans, who will control both chambers in Congress next year, have any intention of passing a bill that the president could sign. They note that Mr. Obama delayed any executive action throughout 2013 and 2014, hoping that Speaker John A. Boehner would allow a vote in the House on a bipartisan bill that passed the Senate.

When that did not happen by the summer, officials said, Mr. Obama decided he should act on his own.

That decision puts the president in a drastically different posture than the one he took in numerous interviews and speeches since 2010. In those settings, Mr. Obama was repeatedly urged to act on his own to reduce the number of families that were being separated by deportations. In each of the appearances, he rejected the idea and urged people to pressure Republicans in Congress to pass a bill.

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