Looking for an instance of bipartisanship in Washington? President Obama has one: Congress's immigration reform effort.

"That’s an example of what we can accomplish when we work on a bipartisan basis," Obama said Wednesday night at a Democratic National Committee fundraiser in Miami.

In one sense, he's right. A bipartisan "Gang of Eight" crafted a bill that has made it to the Senate floor with the potential to win votes from Republicans and Democrats. But viewed another way, the most crucial tests of just how bipartisan the reform effort is are only just beginning.

The "Gang of Eight" at an April news conference. (J. Scott Applewhite/AP)

When lawmakers in both parties are willing to step up and help shepherd bipartisan legislation, it's only part of the battle in Congress. For evidence of this, consider the recent gun control debate.

Advocates of stricter gun laws got what they wanted when Sens. Pat Toomey (R-Pa.) and Joe Manchin (D-W.Va.) stepped forward with an amendment to expand background checks. A Republican sponsor? Check. A Democrat with a strong gun-rights record? Check.

Still, the measure failed to win passage. It was a reminder that the most important tests of compromise and bipartisanship come when the final votes are tallied on the House and Senate floors.

And when it comes to immigration reform, it's still not clear what those final tallies will read. Sen. Marco Rubio (R-Fla.), who is in the "Gang of Eight," has warned that the measure needs tighter border security measures. Even moderate Republican Sen. Mark Kirk of Illinois says he won't support the package without a stronger security component.

Meanwhile, adding too many amendments favored by conservatives could upset Democrats and damage the delicate balancing act the "Gang of Eight" has cultivated for months. And even if the bill passes the Senate, the Republican-controlled House remains a question mark.

For his part, Obama -- who has reinserted himself into the conversation after taking a hands-off approach for months -- was bullish Wednesday that comprehensive reform can win passage soon.

"I actually am pretty confident that before the summer is over, I can sign into law comprehensive immigration reform that will strengthen our borders, fix our legal immigration system, and make sure that those who are here and are undocumented can earn their way -- in an arduous process, but earn their way -- to be full-fledged members of our country," the president said.

If he's right, then a genuine instance of bipartisanship will have occurred. Without Republican support, Obama simply cannot win what he is asking for. And without some Democratic concessions, Republicans will not sign off. This much is clear.

But if the president doesn't get a bill to his desk, it won't matter in the long run that four Republicans and four Democrats got together in an unprecedented era of polarization to craft a bill palatable to both sides. And what that will say is that even a union of pols from opposite ends of the political spectrum like Rubio and Sen. Charles E. Schumer (D-N.Y.) can be stymied.

How bipartisan is any legislative push? The answer lies ultimately in the end result.