WASHINGTON – Senate Republicans and Democrats closed in on a last-minute compromise Thursday on legislation opening the way to legal status and eventual citizenship for many of the 11 million immigrants living in the United States illegally.

President Bush praised the lawmakers’ efforts, noting the details were unfinished, and encouraged them “to work hard and get the bill done.” Sen. John McCain, R-Ariz., said he had been assured the president supports the emerging measure.

As outlined, it would provide for enhanced border security, regulate the future flow of immigrants into the United States and offer legalized status to the millions of men, women and children in the country unlawfully.

“We’ve had a huge breakthrough” overnight, said Majority Leader Bill Frist, R-Tenn.

Sen. Harry Reid of Nevada, the Democratic leader, agreed, but cautioned that the agreement had not yet been sealed.

Even so, the presence of both leaders at a celebratory news conference underlined the expectation that the Senate could pass the most sweeping immigration bill in two decades, and act before leaving on a long vacation at the end of the week.

The developments marked a turnaround from Wednesday, when it appeared negotiations had faltered. The key sticking point involved the 11 million illegal immigrants in the country, and the struggle to provide them an opportunity to gain legal status without exposing lawmakers to the political charge that they were advocating amnesty for lawbreakers.

Although final details were not available, in general, the compromise would require illegal immigrants who have been in the United States between two years and five years to return to their home country briefly, then re-enter as temporary workers. They could then begin a process of seeking citizenship.

Illegal immigrants in United States longer than five years would not be required to return home; those in the country less than two years would be required to leave without assurances of returning, and take their place in line with others seeking entry papers.

Standing before television cameras after an appearance Thursday in Charlotte, N.C., Bush said he was pleased that Republicans and Democrats were working together.

“I appreciate their understanding that this needs to be a comprehensive immigration bill,” hee said. “I recognize that there are still details that need to be worked out. I would encourage the members to work hard to get the bill done prior to the upcoming break.”

Not everyone was satisfied.

“I’m not impressed,” said Sen. Jeff Sessions, R-Ala., who has criticized earlier versions of the measure as too lenient on lawbreakers. Sens. John Cornyn, R-Texas, and Jon Kyl, R-Ariz., joined him in criticizing the measure.

Beyond the illegal immigrants, there were other thorny issues to be clarified.

Senate leaders had yet to unveil draft legislation to make sure that only legal workers were hired in the future, for example.