US Senator Ted Cruz struck first in Saturday's handful of presidential nomination contests, decisively winning Kansas and Maine, and boosting his claim as the most viable alternative to billionaire frontrunner Donald Trump.
On the Democratic side it was Senator Bernie Sanders, the self-described democratic socialist, who claimed the first two victories, in Kansas and Maine. But Hillary Clinton bounced back to easily win Louisiana, seen as the weekend's big prize.
For Republicans, the Saturday 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.
The brash real estate mogul Trump is ahead in the all-important delegate count for the Republicans, having won 10 of the 17 states that have now voted in the process that determines the nominees for both parties. 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 of the vote, doubling up on Trump who received 23.3 percent. Senator Marco Rubio was third at 16.7, followed by Ohio Governor John Kasich with 10.7 percent.
In Maine, it was a startling result for the arch-conservative Cruz in the more moderate New England region. The centrist candidate Romney won Maine caucuses twice, in 2012 and 2008. Republicans were still awaiting results in Kentucky and Louisiana.
Senator Marco Rubio was third at 16.7, followed by Ohio Governor John Kasich with 10.7 percent.
In Maine, it was a startling result for the arch-conservative Cruz in the more moderate New England region. The centrist candidate Romney won Maine caucuses twice, in 2012 and 2008.
Republicans were still awaiting results in Kentucky and Louisiana.
"God bless Kansas!" Cruz told a campaign rally in Idaho, upon learning that he was projected the winner.
"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."
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.
Sanders meanwhile reveled in winning the night's first two races against Clinton.
"It's official: @BernieSanders takes Kansas," the state's Democratic Party posted on Twitter.
While Clinton won just one state Saturday, it was big enough for her to expand her delegate lead as she inches closer to securing the nomination.
The former secretary of state dominated in Louisiana, because of its large African-American vote.
But Sanders did well in the other two states in large part because of their substantial white populations, a demographic with which Sanders has done well.
Maine, also overwhelmingly white, holds its Democratic caucus Sunday.
The races are wedged between more consequential contests: the dozen states that voted on "Super Tuesday" March 1 and the big battles on March 15, when many Republican races, including in Rubio's Florida and Kasich's Ohio, become winner-take-all affairs.
Cruz received an added boost Saturday when he won the straw poll at the Conservative Political Action Conference, a prominent gathering of grass roots far-right activists.
Trump made waves when he cancelled a scheduled Saturday morning appearance at CPAC near Washington, opting instead to hold a rally in Wichita, Kansas.
The move angered members of the American Conservative Union which hosts CPAC.
"I think it was a big mistake for Donald Trump not to be here," ACU chairman Matt Schlapp told CNN.
'Establishment is against us'
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, seen by many political observers as the best hope to defeat Trump, issued a forceful repudiation of the frontrunner, challenging him, like many Republicans have, on his conservative credentials.
Rubio brought 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."
But the first-term senator from Florida was having a rough Saturday night.
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.
There are 155 delegates at stake in Saturday's Republican races.
Heading into Saturday, Trump led the field with 329 delegates, followed by Cruz with 231 and Rubio with 110. Kasich trailed with 25.
Trump has won 10 state contests, while Cruz has claimed six including Kansas and Maine. Rubio has won one.