I will be there tomorrow at the Del Sol High school, among the few live spectators, where Obama will provide more details on the plan.

Here are details of what Obama is going to announce tonight:

President Obama will speak live at 8 p.m. ET from the East Room of the White House.

On Friday, he will travel to Del Sol High School in Las Vegas to further detail his plans and rally supporters. Senate Democratic Leader Harry Reid, D-Nevada, and House Democratic Leader Nancy Pelosi, D-California, will also attend, officials said. The school is the same place where Obama announced a second-term push for immigration reform in Jan. 2013.

The Action

The White House says Obama will “maximize the use of his authority” to extend temporary legal status to more than 5 million undocumented immigrants.

Who Gets Relief?

  • 4.1 million undocumented parents and families of U.S. citizens who have been in country more than 5 years with no criminal record.
  • 300,000 undocumented immigrants who came to the U.S. illegally as children, so-called Dreamers, will be newly eligible for the Deferred Action for Childhood Arrivals (DACA) program. Current age limits for the program will be dropped, sources say.
  • 400,000 highly-skilled workers will be eligible for visas.
  • Some other smaller categories for relief will bring the number affected above 5 million.

Who Gets Left Out?

Border Security

Obama will direct more resources on border security with an emphasis on deporting new arrivals. Guidance to law enforcement will be to focus on criminal aliens rather than those living quietly in the shadows with no arrest record.

The Legal Argument

The White House believes Obama’s acts are effectively bullet-proof in court, backed up by the precedent of more than a dozen presidents who have used discretion in enforcement of immigration law and granted temporary legal status to thousands of immigrants on their watch. Advocates have been told the dreamer families were left out because White House believes inclusion of non-citizen families would jeopardize the legal underpinning of the plan.

The Caveats

Obama will sign his executive order tomorrow at the event in Las Vegas, but it will take several weeks for many of the new initiatives to roll out, people familiar with the plan say. Terms of the action will take effect in six months, giving time for congressional action to replace the order with legislation and allow advocacy groups to organize people to apply for relief.

Nothing in Obama’s plan will create a pathway to citizenship for undocumented immigrants and any legal status received would be temporary. Officials have said the eligible immigrants – as under DACA — would not be entitled to federal benefits such as Medicaid, health care subsidies, etc.

While up to as many as 5 million undocumented immigrants would be eligible to apply for relief under Obama’s new order, it is expected that fewer will actually apply and be approved. So bottom line: 5 million is the high end, but a rough estimate.

The Public View

A majority – 52 percent – of Americans said they would like to see Obama act unilaterally on immigration in the absence of Congressional action, with 44 percent opposed, in an early September ABC News/Washington Post survey. But when asked about plans to extend legal status and work permits to undocumented immigrants now living and working in the U.S., a majority were opposed – 50 percent saying they do not back the idea, up four percentage points from the year before.

ABC News’ Serena Marshall and Devin Dwyer contributed reporting.