* Bush says undocumented immigrants can stay, but only as residents
* Undocumented must return home to apply for citizenship
* Bush says too early to decide if he will be a candidate in 2016 (Adds quotes from Bush interview, political speculation)
By David Adams
MIAMI, March 5 (Reuters) - A new book about immigration reform published on Tuesday by Jeb Bush is fueling speculation that the former Florida Governor may be considering a run at the presidency in 2016.
In the book, Bush seizes the middle ground in a divisive political topic, proposing residency for undocumented immigrants but tough conditions for citizenship.
Bush's plan advocates against allowing undocumented immigrants to obtain citizenship unless they return to their country of origin first, according to the book "Immigration Wars: Forging an American Solution."
Asked by reporters on Tuesday if he is thinking of running in 2016, Bush did not discount the idea, in sharp contrast to his emphatic rejection of a run in the lead up to the 2012 Republican party presidential primaries.
NBC political host Chuck Todd interviewed Bush on Tuesday and told viewers he was "seriously considering" entering the race. Hours later Bush told Reuters, "No, I told Chuck that's not accurate."
He added, "what I have seriously considered is not to consider it seriously for a while. It's so far away," he said.
Journalists were too obsessed with discussing future candidates, he added. "This is like crack cocaine for political reporters, bless their hearts," he said.
Bush's position on citizenship surprised some immigration advocates, as well as fellow Republicans, who had expected him to come out with more lenient proposals.
"I'm surprised and very disappointed," said Cheryl Little, director of Americans for Immigrant Justice, a Miami-based immigrant advocacy group. "It was my impression that Governor Bush has long been a champion of fair and humane immigration reform."
Instead of citizenship, Bush, 60, argues that immigrants who came to the United States illegally could apply for permanent legal residency, as long as they pay a fine and perform community service, as well as paying back taxes and learning English.
Under the Bush plan, undocumented immigrants would be able earn U.S. citizenship only if they return to their home countries and apply through regular legal channels. They would also face a three- or 10-year bar depending on how long they had been in the United States illegally.
Bush makes an exception for young undocumented immigrants, known as DREAMers, who were brought to the United States by their parents before the age of 18. Under his plan they would be given residency with the ability to apply for citizenship after five years.
Senate Majority Leader Harry Reid was quick to seize on the perception that Bush's stance on pathway to citizenship had hardened.
"Let's wait for a few minutes and see how Jeb Bush changes his mind again. His opinion on immigration is not evolving, it's devolving," he said.
Reid, the top Senate Democrat, contrasted Bush's position with that of Florida Republican Senator Marco Rubio who has advocated giving immigrants a more direct pathway to citizenship without being forced to leave the United States, albeit also with strict conditions.
Speaking to reporters in Washington on Monday, Rubio said he had received a copy of the book last week from his longtime political mentor but had not had time to read it.
"I haven't had a chance to talk to him in depth on why he's adopted this new position," Rubio said.
Bush has never previously endorsed a specific package of proposals on immigration reform, though he has on occasion sounded favorable to more generous terms for creating a pathway to citizenship.
Bush said his position has not changed, saying he has long believed in the "fundamental principle that coming to this country legally should be easier than coming illegally."
He said his book's proposals were not meant to be written in stone and were simply a set of recommendations. "There could be other ways to satisfy that principle, so long as it's clear in the law. If not you create a magnet for other people to come illegally," he said.
Bush cited a study published last month by the Pew Hispanic Center that found only 36 percent of Mexicans eligible for citizenship had availed themselves of that option. Mexican immigrants account for 6.1 million - about 55 percent - of the estimated 11.1 million in the United States as of 2011, it pointed out.
"So, based on past history, more than half of the people here illegally would be happy to come out of the shadows and get residency," he said.
Bush also takes a conservative stand in the book over family-based immigration, arguing that the number of relatives who are allowed to immigrate in order to be reunited with family members is too high. Family-based immigration was a strain on the economy because many who are allowed entry are not of working age and "typically do not produce economic benefits," he writes.
"That's so un-American," said Little. "Family unity has long been a cornerstone of American immigration policy," she added. (Additional reporting by Richard Cowan in Washington; Editing by Lisa Shumaker, Bernard Orr)