WASHINGTON - The US Senate faced critical tests Thursday on competing immigration plans, with lawmakers and President Donald Trump divided over how to protect so-called "Dreamers" while tightening rules on immigrants entering the United States.

Congress's upper chamber is considering four different approaches, including a plan laid out by Trump and supported by the Senate's Republican leadership, and a bipartisan measure that appeared to be gaining traction.

The key will be which can receive 60 votes, the threshold for advancing legislation in the closely divided 100-member Senate. Two other measures being considered are not expected to pass.

However, the White House said it would advise President Donald Trump to veto a bipartisan immigration plan, heaping pressure on the US Senate . "If the president were presented with an enrolled bill that includes the amendment, his advisors would recommend that he veto it."

The veto threat pours cold water an what some Republicans were describing as the Senate's most viable immigration proposal, as the chamber struggles to meet a self-imposed end-of-the-week deadline for approving a plan.

Senator Majority Leader Mitch McConnell told AFP he expects "some votes" Thursday, but it remained unclear whether all four options would get votes.

Senate leaders have imposed an end-of-week deadline to protect from deportation some 1.8 million immigrants who were brought to the United States illegally as children.

Their fate has been uncertain since Trump scrapped the Obama-era Deferred Action for Childhood Arrivals (DACA) program last September and gave Congress six months to legislate a solution.

That deadline, March 5, is now just three weeks away, although recent court rulings have made it likely that it would be extended.

Nevertheless, senators see this week as their best chance to break the impasse.

A breakthrough of sorts occured late Wednesday, when eight Republican and eight Democratic senators unveiled a bipartisan plan.

"Our legislation underscores the broad, bipartisan commitment to creating a path to citizenship for Dreamers, who were brought to this country illegally through no decision of their own, while strengthening border security to help stop the flow of illegal immigrants," said Republican Senator Susan Collins.

Like the bipartisan measure, Trump's plan would put 1.8 million Dreamers on a pathway to citizenship, while providing $25 billion for border security, including funding for a wall on the US-Mexico border, which would fulfill a controversial pledge Trump made during his 2016 campaign.

But his plan would also dramatically curtail legal immigration, by ending the decades-old diversity visa lottery and restricting the policy of family reunification, which Trump calls "chain migration." Trump has argued that terrorists have abused the two programs in order to enter the country and kill Americans.

The bipartisan effort would only make limited changes to family reunification, and would leave the diversity lottery untouched.

Republican Senator Lindsey Graham said there was growing consensus around the bipartisan plan. "I think that's got the best chance of getting 60" votes, he said.

For the second straight day Thursday, Trump took to Twitter calling on senators to back his vision instead.

"While the Republicans and Democrats in Congress are working hard to come up with a solution to DACA, they should be strongly considering a system of Merit Based Immigration," he said.

Democrats have panned his proposal, arguing it unfairly restricts legal immigration.

The Senate's top Democrat, Chuck Schumer, had choice words for Trump on the Senate floor, attacking his refusal to compromise on immigration.

"You don't get 100 percent of what you want in a Democracy - maybe a dictatorship," Schumer said.

But the Democrat called on his Senate colleagues to rise above their differences and pass a compromise bill.

"Let's rise to the occasion, and by the end of today let us say to the Dreamers that the Senate believes America has a place for them too."

Number two Republican Senator John Cornyn was sanguine about prospects for the president's plan, saying anything that passes the Senate would need to clear the House of Representatives, where several conservatives are weary of legislation that would give amnesty to immigrants.

"I do think that's the only one that has a path to the president's desk," Cornyn said of the president's plan.

Time is an increasingly important factor.

Some 690,000 Dreamers who registered under DACA - plus 1.1 million others who were eligible but did not sign up - could begin to face deportation beginning next month if no deal is reached.