Senators Ted Cruz and Bernie Sanders scored key victories Saturday in their White House quests, but it was Donald Trump and Hillary Clinton who outperformed their rivals to remain the race's undisputed frontrunners.

Republican Trump and Democrat Clinton did what they needed to do -- dominating in the delegate-rich state of Louisiana in performance that keep them on top at a critical point in the US presidential race.

Results from contests in five states were split, but one element was clear: with Cruz taking Kansas and Maine, he boosted his claim as the most viable alternative to billionaire Trump, and put poorly performing Senator Marco Rubio under immense pressure to turn his campaign around or bow out.

"Thank you to Louisiana, and thank you to Kentucky," Trump said in Florida, minutes after he was also projected to be the winner in Kentucky, where he led Cruz by four percentage points.

For Republicans, the races provide the first tests of whether the establishment's desperate effort to halt Trump, led this week by 2012 nominee Mitt Romney, is having any effect on voters.

Trump declared those establishment efforts a failure, and called on Rubio, seen by many political observers as the best hope to defeat Trump, to pack it in.

"Marco Rubio had a very, very bad night. Personally I'd call for him to drop out of the race," Trump said.

"I would love to be able to take on Ted one on one," he added. "That will be easy."

Beyond the delegate count, Cruz and Sanders can claim momentum as they head toward critical races in Michigan next Tuesday and then winner-take-all races in the large states of Florida and Ohio on March 15.

The brash real estate mogul is ahead in the all-important delegate count for Republicans, having now won 12 of the 19 states that have voted since Iowa kicked off the race last month.

But Cruz's wins are a reminder that while Trump still appears to be the likely nominee, it is by no means inevitable.

The conservative senator performed beyond expectations in Kansas, where he earned 48.2 percent support, doubling up on Trump who received 23.3 percent.

Rubio was third at 16.7, followed by Ohio Governor John Kasich with 10.7 percent.

In Maine, it was a startling 13-point win for the arch-conservative Cruz in the more moderate New England region.

Centrist candidate Romney won Maine caucuses twice, in 2012 and 2008, but it was a disaster for the current establishment favorite Rubio, who finished fourth there Saturday.

Cruz exulted in his victories during a campaign rally in Idaho.

"The scream you hear -- the howl that comes from Washington, DC -- is utter terror at what we the people are doing together," he said, adding that conservatives are "coming together... and standing as one behind this campaign."

- Momentum -

The Republican race has been winnowed to four men: political outsiders Trump and Cruz, and more mainstream candidates Rubio and Kasich. Many in the Republican establishment are apoplectic over whether anyone can stop Trump's march.

On the Democratic side, it was self-described democratic socialist Sanders who savored victories in Kansas and Nebraska, pushing his total to seven of the 18 Democratic contests.

"We've got the momentum, the energy and the excitement that will take us all the way to the Democratic National Convention in Philadelphia," Sanders said in a statement.

But Clinton decisively swept Louisiana, seen as the weekend's big prize, with 59 Democratic delegates at stake compared to 37 for Kansas and 25 for Nebraska.

The former secretary of state dominated in Louisiana, with its substantial African-American vote.

Sanders did well in the other two states in part because of their substantial white populations, a demographic with which Sanders does well.

Maine, also overwhelmingly white, holds its Democratic caucus Sunday.

Trump made waves when he cancelled a scheduled Saturday appearance at the Conservative Political Action Conference near Washington, opting instead to hold a rally in Wichita, Kansas.

Trump told the Wichita crowd that Romney, who on Thursday called Trump "a fraud," was a "loser" who should have defeated President Barack Obama.

"It's the establishment. The establishment is against us," Trump said.

Rubio issued a forceful repudiation of the frontrunner, bringing the house down at CPAC when he warned about a dire future for Republicans "if the conservative movement is hijacked by someone that's not a conservative."

With Trump's challengers insisting they are in it for the long haul, there is a chance no candidate will rack up the 1,237 delegates needed to secure the nomination before the convention in July.

That would mean a contested or "brokered" convention, a scenario that could turn chaotic, especially if establishment figures seek to somehow actively prevent delegates from coalescing around Trump.

Including partial delegate results from Saturday, Trump led the field with 373 delegates, followed by Cruz with 291, Rubio with 122 and Kasich 33.