WASHINGTON -- U.S. President Donald Trump warned Tuesday that it's now or never when it comes to extending protections for young immigrants, while Senate Majority Leader Mitch McConnell threw his weight behind legislation based on the president's priorities.

said Congress must act now to provide legal protections to young "Dreamer" immigrants even as legislation faces an uncertain prospect in Congress.

"Wouldn't it be great if we could finally, after so many years, solve the DACA puzzle," he wrote, adding: "This will be our last chance, there will never be another opportunity! March 5th."

Trump was referring to a deadline he announced last year to end a program protecting young immigrants from deportation. But a recent court ruling has rendered that deadline all but meaningless.

The comments came the day after the Senate voted 97-1 -- Ted Cruz, R-Texas, provided the sole "no" vote -- to plunge into an open-ended immigration debate that's been promised by McConnell. Both parties' leaders hope debate can be concluded this week, but it's unclear if that will happen or what the product, if any, will be.

One GOP proposal would pave a path to citizenship for up to 1.8 million young "Dreamer" immigrants in the U.S., a lure for Democrats that many Republicans oppose.

Trump also wants $25 billion for Trump's border wall with Mexico and other security measures, as well as curbs on legal immigration -- a must for many Republicans.

"This proposal has my support, and during this week of fair debate, I believe it deserves the support of every senator who's ready to move beyond making points and actually making a law," McConnell said in beginning Senate debate Tuesday.

McConnell and other GOP supporters describe the measure as the Senate's best shot of passing a bill that the president will sign, but many Democrats consider some of the proposals, including limiting the relatives that legal immigrants can bring to the U.S., to be non-starters.

Leading up to the debate, the Senate's two top leaders put on a show of camaraderie, but also laid down markers underscoring how hard it will be to reach a deal that can move through Congress.

"We really do get along, despite what you read in the press," Democratic leader Chuck Schumer of New York said Monday at a previously scheduled appearance alongside McConnell at the University of Louisville.

Still, just days after the two leaders brokered a bipartisan $400 billion budget agreement and helped shepherd it into law, both men made clear that an immigration agreement will be tough.

Schumer has expressed repeated opposition to Trump's sweeping approach, pushing for a more narrow focus.

"Helping the Dreamers become Americans and protecting our borders. That should be the focus of all our energies," Schumer said Tuesday.

Trump's overall immigration plan, opposed by many Democrats, stands little chance of prevailing because any measure will need 60 votes. That means proposals will need substantial bipartisan support since the GOP majority is 51-49, and Sen. John McCain, R-Ariz., has been absent in recent weeks battling cancer.

Sen. Tom Cotton, R-Arkansas, one of the lawmakers who introduced a bill based on Trump's plan, described it as a "best and final offer" and said there's no room for negotiations.

He told Fox News' "Fox and Friends" Tuesday morning that Congress has just two options: "the president's framework bill or nothing."

Sen. Jeff Flake, R-Arizona, also proposed a modification of the bill late Monday that he painted as a potential compromise.

Highlighting the partisan gap, there was plenty of finger-pointing on Monday.

Trump put the onus on Democrats, saying, "I hope the Democrats are not going to use it just as a campaign" issue.

The Senate's second ranking Democrat, Dick Durbin of Illinois, said the key impediment to a bipartisan deal was Trump's history of switching positions on the issue.

"Nailing the president down has been next to impossible," he said.

Meanwhile, Trump made clear he continues to take his March 5 deadline seriously, even though because of a judge's ruling, federal immigration officials have been renewing permits under President Barack Obama's Deferred Action for Childhood Arrivals, or DACA, program for immigrants' whose status expires beyond that date.

"Negotiations on DACA have begun. Republicans want to make a deal and Democrats say they want to make a deal," Trump wrote even though negotiations have been underway for months.

------

Associated Press writers Kevin Freking and Jill Colvin in Washington and Bruce Schreiner in Louisville, Kentucky, contributed to this report.