In a thinly veiled threat to congressional Republicans, President Obama on Tuesday warned the GOP not to obstruct a comprehensive immigration-reform bill now advancing in the Senate, ending his virtual silence on an issue paramount to his second-term agenda.

"If you're actually serious and sincere about fixing a broken system, this is the vehicle to do it -- and now is the time to get it done," Obama said from the East Room of the White House. "There is no good reason to play procedural games or engage in obstruction just to block the best chance we've had in years to address this problem."

Obama called for lawmakers to reach agreement by summer's end on an immigration fix, one version of which easily cleared a key, test vote in the Senate Tuesday.

But House Speaker John Boehner, R-Ohio, whose Republican members are among the most vocal opponents of the reform efforts, isn't in as much of a rush. He said a solution could be made by year's end.

Obama has generally steered clear of lawmakers' negotiations over immigration in recent months, hoping not to alienate conservatives who might back a provision he seeks to provide a pathway to citizenship for the nation's 11 million illegal immigrants. But Obama changed tactics Tuesday, dismissing GOP charges that the bipartisan compromise amounted to amnesty for lawbreakers.

It remains to be seen how Obama's re-emergence after months on the sidelines will shape the immigration debate. Some liberals had asked him to stay out of the way so Republicans could avoid charges that they were rubber-stamping Obama's second-term agenda.

And Republicans opposed to the compromise now moving ahead in the Senate told The Washington Examiner that Obama's challenge was a welcomed gift, saying his overtures right now are so toxic they undermine his cause.

"We love it when the president trumpets immigration reform," said an aide to a Republican senator opposed to the bill. "The more he says, the more it helps us. Keep it coming."

The White House is desperate to talk about subjects other than the administration's massive Internet and phone surveillance programs or the IRS targeting of conservatives groups or the Justice Department monitoring of journalists -- each overlapping controversy fanning flames about government overreach.

Obama filled that vacuum just hours before senators voted 82-15 to open debate on the immigration bill. In the days and weeks that follow, lawmakers will offer a series of amendments and hold conferences on a piece of legislation with more long-term ramifications than arguably any bill since Obama's health care overhaul.

Tuesday's procedural Senate vote in no way guarantees smooth sailing for the legislation.

"This bill is going to pass the Senate, but as written, this bill will not pass the House," Sen. Ted Cruz, R-Texas, predicted Tuesday.

In essence, Obama was hoping to paint opponents of the compromise bill as out of step with the majority of Americans.

Cognizant of the bill's greatest vulnerabilities, Obama talked tough about border security amid Republican complaints that the immigration-reform package has no clear system for measuring whether the border is secure, saying such reforms were "no cakewalk" for those looking to obtain citizenship.

"Illegal crossings are near their lowest level in decades," Obama insisted. "Nobody is taking border enforcement lightly."