Barack Obama has been urged by the majority of Democrats in the House of Representatives to use his presidential authority to enact the kind of “bold and meaningful” reforms that will protect millions of undocumented migrants from deportation.

The direct plea from the rank and file of Obama’s own party was made as the White House finalises its plans for a widely anticipated executive order that would provide work permits to almost half of the 11 million people living in the country illegally.

Obama has pledged to push ahead with executive action despite trenchant opposition from Republicans who argue their triumph in last week’s midterm elections should force the president to put the brakes on his plans.

However in a letter to Obama released on Thursday, 116 House Democrats said it was clear Republican leaders would not pass legislation to enact comprehensive immigration reform. “By failing to do their job – and repeatedly interfering with your efforts to do your job – congressional Republicans threaten to take our immigration system hostage and preserve a status quo that everyone agrees is unacceptable,” the letter said.

“Their failure to act must not inhibit your commitment to governing.”

A presidential overhaul of the immigration system will put the White House on a collision course with Republicans, who swept to victory in dozens of key races in the midterm elections, enabling the GOP to retake control of the Senate from January.

The Republican speaker of the House, John Boehner, warned Obama earlier this week he would be “inviting big trouble” by using his presidential authority to reform the immigration system, setting the scene for the first major collision between the parties after the midterm elections.

But the presidential order, which is expected to build on a 2012 measure that protected young people brought to the country illegally as children, will be a huge victory for Latino and immigration rights groups and liberals in his party.

One senior Democratic source told the Guardian that Obama was expected to make his move after the 6 December Senate election runoff in Louisiana, a state likely to react badly to any move by Democrats to shield undocumented workers from deportation.

Details of the proposed executive order remain thin. Two reports – by the New York Times and Fox News, both of which cited administration officials with knowledge of a draft plan – said the president was considering expand the deferred action order which mandated immigration authorities to suspend deportations of young people brought to the country illegally.

The deferred action against childhood arrivals (DACA) order, which looks set to broaden to make a larger group of people eligible, also gave applicants work permits.

Crucially, both reports said Obama would also order immigration authorities to suspend deportations of parents of US citizens and legal residents, a measure that could benefit some 3.3 million undocumented migrants living in the country illegally.

The White House did not immediately respond to a request for comment on the reports but administration officials have in recent days sounded bullish about the president’s intention to pursue the executive action, which was postponed until after the midterm elections, despite Republican opposition.

Obama’s chief press secretary Josh Earnest told reporters traveling with the president in Burma on Thursday that no final decision had been taken about what would be contained in the order. However, he indicated a decision would be taken soon after Obama returns from his trip to Asia. Both reports said the order would be signed next week.

Earlier on Thursday, Illinois congressman Luis Gutiérrez, a leader of the pro-reform movement, told reporters: “I am here with dozens of Democrats and what we want the president to do is to act boldly, act swiftly, and act generously. We want millions of Americans to know that their Thanksgiving is not going to be interrupted by an immigration agent.”

Gutiérrez insisted he had not got any inside track on what the White House was or was not planning, despite media reports that action was imminent.

“I have not heard what the New York Times and Fox News think what they have heard about what the president might be going to do,” he said.

But several of those present, who are known to be close to the White House, spoke in the past tense about the decision and began preparing the arguments against possible recriminations and drawing parallels with the civil rights struggle.

“After the president exercises his broad authority to defer removals, we will hear complaints that he acted outside the law,” said Michigan representative John Conyers, a senior member of the judiciary committee.

“But like those before him, president Obama’s legal authority to act derives from the constitution and the immigration and nationality act itself. The only path forward at this time is through executive action.”

Zoe Lofgren of California added: “Many of our Republican colleagues say president doesn’t have authority or will poison the well if he does this; well they are wrong.”

“The president can’t just wait,” she said, claiming the the chance of a Republican Congress passing its own immigration bill was an “unlikely proposition”.