Speaking in Las Vegas, President Obama laid out his plan for immigration reform, which includes smarter enforcement, a clearer path to citizenship and improvements in the legal system. (The Washington Post)

Rising tensions over whether to give illegal immigrants a chance to pursue full citizenship could ruin what President Obama and congressional leaders agree is a pivotal moment in resolving long-simmering problems in the country’s immigration system.

Immigrant advocates and their Democratic allies insist that now, at long last, is their time. After various failed proposals over the past decade, they finally feel they have the leverage to accept nothing less than a path to full citizenship for the millions of people living illegally in the country.

But although Republican leaders are newly interested in a compromise on immigration, many in the party say that allowing undocumented immigrants to live here legally is enough and that a push for citizenship would face fierce, and possibly insurmountable, opposition from conservatives.

The tension has deepened in recent days, with disagreements emerging within each party as bipartisan groups in the House and the Senate try to move toward a compromise even as they face hard-line opposition from some voices in their political bases.

On the right, some conservatives have begun heaping criticism on one of their own rising stars, Sen. Marco Rubio (R-Fla.), the Cuban American who is a potential presidential candidate and who is championing a compromise. On the left, some liberals are privately grousing that Democratic senators working with Rubio are giving too much ground.

In a speech in Las Vegas, President Obama described guidelines for immigration reform a day after a group of senators made their own proposal. White House correspondent David Nakamura discusses the timing of the announcements and the prospects for a deal. (The Fold/The Washington Post)

A key question is whether to require that certain conditions be met before illegal immigrants could be put on the path to citizenship — and how the government would determine success.

The Senate group, which includes Rubio and top members of both parties, would require that the U.S.-Mexico border be found secure and that other strict enforcement measures be enacted before those here illegally could become citizens. Many on the left say the path needs to be more straightforward, while many on the right see even the compromise idea as a non-starter, deeming it too lenient.

A path to citizenship is “certainly going to be a problem in the House,” said Rep. Bob Goodlatte (R-Va.), chairman of the Judiciary Committee, which will hold a hearing next week on the issue. “There are a lot of options between deporting 11 million people, which most people don’t believe will happen, and giving [them] citizenship.”

On the other side, Richard Trumka, president of the AFL-CIO, said he would support only legislation that gives every deserving illegal immigrant a chance at citizenship. “If it’s too exclusionary, then we’ll fight against it,” he said.

The tensions underscore the difficulty of forging consensus on such a politically charged issue, even after Obama’s decisive election win last year among Hispanics led several prominent Republicans to express an eagerness to strike a deal.

The senators behind the framework — Republicans Rubio, John McCain (Ariz.), Jeff Flake (Ariz.) and Lindsey O. Graham (S.C.), along with Democrats Charles E. Schumer (N.Y.), Richard J. Durbin (Ill.), Robert Menendez (N.J.) and Michael F. Bennet (Colo.) — have been exuding confidence that a deal was within reach.

“I’ve never felt more positive about the prospects of real immigration reform than I do today,” Durbin said at a news conference Thursday.

Yet even if the senators find agreement among themselves, selling their recommendations to their colleagues and the activists on both sides of the debate will be a far steeper challenge.

Immigration advocates close to the White House have vowed to pressure Obama if he agrees to what they consider unreasonable preconditions to citizenship for illegal immigrants. Conservatives are either insisting on strict contingencies or refusing to back the idea of citizenship.

“The world now thinks that this is inevitable,” said one person with knowledge of the deliberations who spoke on the condition of anonymity. “This is far from inevitable. There’s a million land mines in the way.”

Obama has welcomed the Senate plan. But in a speech this week, he called for a path to citizenship “from the outset,” and in subsequent interviews, he appeared to draw a clear line on the issue.

“What we don’t want to do is create some kind of vague prospect in the future that somehow comprehensive immigration reform that includes a pathway to citizenship will happen, you know, mañana,” Obama told Univision anchor Maria Elena Salinas in a White House interview aired late Wednesday, using the Spanish word for “tomorrow.”

Obama is under pressure to deliver on citizenship from supporters who believe they made his reelection possible. Moreover, many Hispanic leaders think that in his first term, Obama broke a promise to pursue an immigration overhaul. Some advocates remain wary that the president and Democratic lawmakers might be tempted to bargain away their best hope for a clear citizenship path in their quest for a bipartisan deal.

The Senate group stoked that wariness with one of the provisions in its citizenship plan: the creation of a special commission, made up of border-state governors and other local officials, to help determine whether the border is secure and, therefore, whether immigrants can gain the right to pursue citizenship.

Such a consensus would be difficult, if not impossible, immigrant advocates argue. For instance, many conservatives say the border remains a problem despite the deportation of more than 1 million immigrants and unprecedented spending by the Obama administration. Many experts, on the other hand, believe the border is effectively secured.

Some liberals wondered whether the new commission would empower conservatives like Arizona Gov. Jan Brewer (R) to complicate the citizenship process. The senators later clarified that the Department of Homeland Security would have the final say, drawing on objective data, in addition to advice from the commission — but some advocates viewed the commission idea as a tactical error by Democratic negotiators.

“It pushed the debate to the right more than it needed to be,” said one advocate close to the deliberations.

Arturo Carmona, executive director of the group Presente.org, a more combative pro-immigrant group on the left, said he was “shocked” at how the discussion in Washington had shifted in the immediate aftermath of the 2012 elections, when it appeared the momentum was on the side of the pro-immigrant groups. He warned that liberal groups and the White House risked losing their leverage if they compromise.

“It feels like we’ve gone back in a time machine, ignoring everything that’s happened,” he said.

The flare-ups on the right have put Rubio on the front lines facing a GOP base that until now has seen him as a hero for conservative causes.

The National Review — which featured Rubio, then a dark-horse Senate candidate, on its cover in 2009, boosting his stature in the conservative movement — singled him out for criticism in a Wednesday editorial, calling him “wrong about how to go about repairing our immigration system.” The editorial is headlined “A Pointless Amnesty.” A fellow Republican senator, David Vitter of Louisiana, described Rubio on a radio show as “amazingly naive” on the issue of immigration reform.

Rubio has tried to coax conservatives along by assuring them that any deal would include strict conditions and enforcement requirements.

“If there is not language in this bill that guarantees that nothing else will happen unless these enforcement mechanisms are in place, I won’t support it,” he told talk radio host Rush Limbaugh this week.

Writing this week on the conservative Web site Red State, which criticized the bipartisan Senate plan, Rubio said conservatives had secured “important concessions” from Democrats, such as the border security contingency for citizenship and a program to verify that employers are hiring legal workers.

Rubio’s presence in the talks gives cover to skeptical Republicans. But Republicans could face a dilemma if Obama and congressional Democrats reject the contingencies and force them to vote on a more direct citizenship path.

The citizenship question has “always been the hardest needle to thread on this issue,” said a senior GOP House aide, requesting anonymity to discuss internal thinking. “If the bipartisan Senate group thinks they’ve found a way to thread that needle, then the White House needs to be very careful not to screw it up.”

Another ominous sign for backers of the citizenship path is the response in recent days of Rep. Raúl R. Labrador, an Idaho Republican and Puerto Rico native who was elected with tea party support and has been seen by immigration advocates as a possible bridge to House conservatives. Labrador, a former immigration lawyer, said in an interview that a citizenship plan would be “difficult for the House to accept.”

He called a potential Republican willingness to accept legal residency for illegal immigrants a “huge shift” and said Democrats will need to similarly back off a demand for citizenship.