A day after the U.S. Senate rejected a series of gun control plans backed by President Obama, Democratic leaders decided to put that work on hold and move on to other legislation, unable to muster enough votes to push gun plans past Republican opposition.

"This debate is not over," said Senate Majority Leader Harry Reid on the Senate floor, as he vowed to bring the bill back in coming months for more work on gun control.

"In fact, this fight is just beginning," said Reid.  "I've spoken with the president. He and I agree that the best way to keep working towards passing a background check bill is to hit a pause and freeze the background check bill where it is."

But the bottom line right now is simple - gun control legislation has run aground in the U.S. Senate.

Reid immediately moved to start Senate work on a bill that would allow states to collect sales taxes on internet purchases, hoping to finish that by next week.

After that, Democrats hope to move to immigration reform legislation in May, which Democrats acknowledge has a much broader level of support in both parties than the gun control measure.

While the gun control bill was not technically "defeated" in the Senate, it made little headway in the last two weeks, as opponents refused to agree to a bipartisan plan on background checks worked out by Sen. Joe Manchin (D-WV) and Sen. Pat Toomey (R-PA).

Reid said work on that background check issue would continue, as he urged advocacy groups to keep working for gun control and to keep putting political heat on opponents.

"I'm committed to ensuring that any bill we pass include an expansion of background checks, close the gun loophole, gun show loophole and to cover private sales," said Reid, as other Senators vowed to keep working as well.

"Yesterday was one of the saddest, most troubling days in my 40-plus year career in public service," said Sen. Richard Blumenthal (D-CT), who said he could not look the families of Newtown victims in the eye and tell them nothing would be done on gun violence.

Before Democratic leaders moved off the gun control bill, Senators approved two amendments to the bill, one that bolsters mental health treatment in education and another that would penalize states if local governments release information on gun ownership.

22 Democrats voted for that measure, yet another reminder that gun rights supporters can tap a number of votes on the Democratic side at times, one reason why this underlying gun control bill was in tough straits well before the debate even began.

As for when gun control might return to the Senate floor, Sen. Reid did not give any hint, though some speculate that the mid-term elections of 2014 might play a role in that schedule.