By John Whitesides and Amanda Becker

WASHINGTON (Reuters) - U.S. Republican presidential candidate Ted Cruz gained momentum on Saturday, projected as the winner of the nominating contest in Kansas and the winner of a "straw poll" of conservative activists.

Cruz, currently in second place in a Republican field that has been winnowed to just four, is seeking to dent the lead of brash New York billionaire Donald Trump. Trump and Democrat Hillary Clinton are each looking to strengthen their front-runner status as five states hold presidential nominating contests on Saturday.

Trump has a substantial lead in the delegates needed to secure the nomination at the Republican National Convention and is trying to weather a barrage of blistering attacks from his party's establishment as Republicans vote in Kansas, Kentucky, Louisiana and Maine.

Polling has been scarce in all four states, which together account for just 155 delegates, and the four contests are open only to registered Republicans. The exclusion of the independent voters who have helped Trump's surge adds an air of uncertainty to the latest round of state-by-state contests to pick nominees for the Nov. 8 election to succeed President Barack Obama.

Both CNN and MSNBC projected Cruz as the winner in Kansas.

Saturday's contests were the first since retired neurosurgeon Ben Carson dropped from the race, after polling in the single-digits in most of the nominating contests. Carson had drawn support from evangelical voters, a group that has also been a stronghold of Cruz.

Since winning seven of 11 contests on Super Tuesday, Trump has come under withering fire from a Republican establishment worried he will lead the party to a resounding defeat in November's election.

Mainstream Republicans have blanched at Trump's calls to build a wall on the border with Mexico, round up and deport 11 million undocumented immigrants and temporarily bar all Muslims from entering the United States.

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Mitt Romney, the 2012 Republican presidential nominee, called Trump a phony and a fraud who was playing American voters for suckers, and 2008 nominee John McCain, a U.S. senator from Arizona, said Trump's foreign policy views were uninformed and dangerous.

At the Conservative Political Action Conference (CPAC) on Saturday, held near Washington, D.C., U.S. Senator Marco Rubio of Florida, who is also challenging Trump, criticized the media for fueling his rise.

“I’ve been sitting here for five minutes and two of the three questions you’ve asked have been about Donald Trump,” Rubio said during a question-and-answer session with CNN's Dana Bash.

Cruz, a U.S. senator from Texas, made a similar argument related to the media in Wichita, Kansas, on Saturday. CPAC attendees chose Cruz as their preferred candidate in a "straw poll" on Saturday.

Rubio, a frequent target of Trump's attacks, also told Bash he "didn't get into this race to beat up on other candidates." But, he added, "If someone keeps punching people in the face, eventually someone's going to have to stand up and punch him back."

At a rally in Rubio's home state on Saturday, Trump criticized his opponent, using a favorite nickname - "little Marco."

"He's a nasty guy, said nasty things, and you know? We hit him hard. Little nasty guy," Trump told the cheering crowd.

The anti-Trump forces have a short window to stop the caustic businessman, who has accumulated 319 of the 1,237 delegates needed to win the nomination at July's Republican national convention, outpacing Cruz, who has 226 delegates.

On March 15, the delegate-rich states of Florida, Illinois, Ohio, Missouri and North Carolina will vote. Both Florida and Ohio use the winner-take-all method to allocate Republican delegates, making the stakes in those two states particularly high. If Trump takes Florida and Ohio he would be nearly impossible to stop. There are a total of 358 delegates at stake in the five states voting March 15, including 99 in Florida and 66 in Ohio.

On the Democratic side, voters in Louisiana, Kansas and Nebraska were weighing in on Saturday on the race between Clinton, the former secretary of state, and Bernie Sanders, a U.S. senator from Vermont.

Polls show Clinton with a big lead in Louisiana, which has a large bloc of African-American voters, who helped her roll up victories across the South on Super Tuesday. But the caucuses in less diverse Kansas and Nebraska could be more suited to Sanders. The three states have a total of 109 delegates at stake.

Clinton has opened up a big delegate lead and Sanders might have a tough time making up the difference. All states in the Democratic race award their delegates proportionally, meaning Clinton can keep piling up delegates even in states she loses.

(Additional repoerting by Emily Flitter and Alana Wise; Editing by Richard Valdmanis and Leslie Adler)