(Reuters) - Immigration activists close to the White House worry that President Barack Obama could delay or scale back executive actions on immigration that he has promised to take before the year ends.

Advocates have pressed the Obama administration to provide relief from the threat of deportation to more than 5 million undocumented immigrants but fear, after some were briefed by administration officials, that the plan could be reduced to 3 million or fewer, a significant drop.

"There's growing nervousness that instead of going big and bold that the administration might play it cautiously," said Frank Sharry, executive director of advocacy group America's Voice.

The fears are rooted in politics and a history of perceived broken promises.

Advocates worry the president might be less aggressive if Republicans take over the Senate in Tuesday's congressional elections. Republicans have vowed to pass legislation to prevent Obama from implementing the planned actions.

The president could remove the deportation threat for about 3 million undocumented immigrants who have lived in the country for 10 years and have children who are U.S. citizens. But activists want the parents of so-called Dreamers, children who have already been granted deportation relief, to be covered too.

"Ultimately it is about political will," said Marielena Hincapie, executive director of the National Immigration Law Center.

She said Department of Homeland Security officials had briefed her and other advocates about a scenario in which 2-3 million people were covered and one in which closer to 5 million were covered.

"They are more likely to take a more cautious approach that they think will be palatable to both Republicans and Democrats, but also probably to the American public," Hincapie said.

The White House said Obama had not made a decision yet and that final recommendations from Attorney General Eric Holder and Homeland Security Secretary Jeh Johnson were pending.

"We expect to make an announcement about these decisions before the end of the year," said White House spokeswoman Katherine Vargas, seeking to tamp down concern about a delay.

"It is premature to speculate about the specific details including the scope or number of immigrants who will benefit since final recommendations from Secretary Johnson and Attorney General Holder, and final decisions by the president, have not been made," she said.

Obama put off his reform plan last month because of concern that it would hurt Democrats running in the November elections.

It may not be clear on Tuesday which party will control the Senate because tight races in Louisiana and Georgia could trigger run-off elections. Advocates fear that the White House might postpone action if that is still unclear by the end of the year.

"It depends on the outcome of the election," said Angela Kelly, an immigration specialist at the Center for American Progress, which has close ties to the White House.

"It's more likely to be a December holiday surprise or holiday gift."

About 11 million undocumented immigrants reside in the United States. Obama has promised to implement broad reform of the U.S. system but has been unable to get Republican support in the House of Representatives for a new law.

Another delay could hurt his legacy and spur criticism from potential 2016 Democratic presidential candidates such as Hillary Clinton.