Speaking at the White House on Tuesday morning, President Obama urged Congress to pass a bipartisan bill that would overhaul the country’s immigration system. (The Washington Post)

President Obama, his second-term agenda bogged down amid political controversies and partisan gridlock, moved Tuesday to bolster support for a sweeping overhaul of immigration laws, saying Congress “needs to act and the moment is now.”

Obama’s speech in the East Room of the White House marked his most forceful remarks on the issue in weeks and came shortly before the Senate voted 84 to 15 to bring a comprehensive immigration bill to the floor for debate. Though all 55 Democrats and a majority of Republicans supported moving the bill forward, the legislation faces a far-from-certain path in the Senate, which is likely to vote in early July.

After months of allowing a bipartisan group of eight senators to take the lead on drafting the bill, Obama sought to reassert his position at a time when conservative Republicans have begun mounting a forceful opposition.

The president said the 867-page bill is a compromise in which neither Democrats nor Republicans would get everything they want. But he emphasized that the United States is a “nation of immigrants” and said the estimated 11 million people who are in the country illegally deserve a clear chance to become citizens.

“If you genuinely believe we should fix our broken immigration system, there’s no good reason to stand in the way of this bill,” Obama said, flanked by a coalition of business, religious and political leaders. “If you’re serious about actually fixing the system, then this is the vehicle to do it.”

By reentering the debate this week, the president is trying to exert his influence while not upsetting the delicate balance of the bipartisan group that negotiated the deal, aides said.

Obama has made the immigration overhaul one of his top second-term priorities, but his administration played mostly a supportive role as the Senate group negotiated the legislation. White House aides have said the president recognizes that being too far in front on immigration could risk scaring off Republicans fearful of being tied too closely to him.

But with the floor debate opening in the Senate, Obama’s return to the spotlight comes at an important moment. Republican critics of the bill have begun championing a series of amendments to further strengthen border security, which immigration advocates fear would slow or even block the path to citizenship.

Meanwhile, outside organizations on both sides have stepped up their lobbying campaigns. The Heritage Foundation, a conservative think tank, announced Tuesday that it has launched a $100,000 online campaign in opposition to the bill. The Services Employees International Union, which supports the bill, announced a seven-figure campaign featuring commercials on national cable channels.

Senate proponents of the legislation have said they hope to win up to 70 votes to pressure the GOP-controlled House to accept the major components of the bill. Advocates also said they were encouraged by Obama’s return to the issue after months of focusing on gun control, the budget and foreign policy.

In his speech, the president highlighted a broad swath of support for the bill, appearing with AFL-CIO President Richard Trumka, U.S. Chamber of Commerce President Thomas Donohue, former George W. Bush administration commerce secretary Carlos Gutierrez, business executive Steve Case, San Antonio Mayor Julian Castro (D) and others.

“Now we’re in another phase. He needs to twist some arms, including some in his own party,” said Kevin Appleby, director of migration policy at the U.S. Conference of Catholic Bishops, which supports the bill. “He set down some markers. He pushed back a bit on the pending amendments. He made a case for the bill.”

Critics pushed back quickly as the Senate began debating the legislation. Sen. Ted Cruz (R-Tex.) said that while he expected the legislation to pass the Senate, it would fail in the House unless proponents agreed to additional border security measures and backed off a guaranteed path to citizenship for illegal immigrants.

“The path to citizenship is the single most divisive issue,” Cruz said. “It’s an issue that the Obama White House and Senate Democrats are insisting on, and by insisting on that I believe they have designed a bill that is destined to be voted down.”

Over the next several weeks, senators on both sides of the aisle are expected to offer dozens of amendments. Senate Minority Leader Mitch McConnell (R-Ky.) said the bill has “serious flaws” but supported allowing it to go to the floor for debate. McConnell was one of several leading Republicans who said they would not support the legislation unless additional border security provisions were added.

As Obama spoke at the White House, Sen. John Cornyn (R-Tex.) laid out his proposal to require 100 percent border security before any undocumented immigrants are eligible for legal status.

Senate Majority Leader Harry M. Reid (Nev.) and other Democrats have described Cornyn’s proposal as a “poison pill” amendment that would unravel the bipartisan alliance.

“I believe the opposite is true,” Cornyn said. “If we don’t guarantee results on border security, if we don’t guarantee to the American people that we actually are going to get serious about stopping the flow of people illegally crossing our northern, west or southwestern border, that is the real poison pill.”

Sen. Marco Rubio (R-Fla.), a key member of the bipartisan group, said he believes many of his GOP colleagues will support the bill if amendments are made that give Congress more authority over measures that the Department of Homeland Security would be required to implement along the U.S.-Mexico border. But he stopped short of endorsing Cornyn’s amendment.

Obama said his administration has invested heavily in border security measures, noting that attempted border crossings have fallen to historically low levels and that deportation rates are at record highs.

“Nobody’s taking border enforcement lightly,” the president said.

Ed O’Keefe contributed to this report.