By James Oliphant

WASHINGTON, Feb 23 (Reuters) - Liberal Democratic presidential contenders' rush to embrace the left's most ambitious proposals has some Democrats worried there could be a price to pay when they try to defeat President Donald Trump next year.

Party activists have been energized as Cory Booker, Kirsten Gillibrand, Kamala Harris and other candidates endorsed plans to provide Medicare coverage to every American, some form of tuition-free college, a national $15 minimum wage and the so-called "Green New Deal" advocated by U.S. Representative Alexandria Ocasio-Cortez.

But Trump and his allies in the Republican Party have seized on those stances to attack the Democratic 2020 field as outside the American political mainstream — a claim the president plans to make throughout his re-election campaign, according to sources with knowledge of his strategy.

Some Democrats fear the argument has potency. They worry the primary may produce a nominee who will not appeal to centrist working and middle-class voters who voted for Trump in 2016 but whom Democrats believe they can win back.

"The big progressive programs are popular in a caucus or primary electorate, but probably don't move the needle among voters who want to find someone who will change Washington by tilting the system to favor people in the middle — not the very rich or the very poor," said Jeff Link, an Iowa Democrat who worked for former President Barack Obama's campaign.

A person familiar with the president's thinking told Reuters that Trump had been looking for a "big contrast issue" to help power his 2020 bid.

His last Democratic opponent, Hillary Clinton, was widely known to the voting public before her campaign. This time, Trump may face someone new to the national stage, and he is looking to brand that candidate before she or he emerges as the nominee.

In recent speeches, including his State of the Union address and again this week in Florida, a key 2020 battleground, Trump used the crisis in Venezuela to equate Democrats with socialists.

"There's no question this is a deliberate strategy on his part," said Matt Bennett, a political analyst with Third Way, a Democratic centrist think-tank. "It is a bit scary to think about what it could do to us in a close, tough election next year."

GOING GREEN

Democrats have already seen the risks of catering to progressives.

Senators Booker of New Jersey, Harris of California, Gillibrand of New York and Elizabeth Warren of Massachusetts almost immediately backed Ocasio-Cortez's push earlier this month for the Green New Deal, a sweeping 10-year blueprint for combating climate change that involves reducing carbon emissions and retrofitting infrastructure.

Senator Bernie Sanders, a Democratic socialist who announced this week he is running for president a second time, plans to introduce his own version of the climate plan.

Ocasio-Cortez, who has enjoyed disproportionate influence for a first-term congresswoman because of her social media presence, was forced to backtrack when an information sheet contained policy goals not in the plan, including doing away with nuclear power and airplanes and providing income to Americans "unwilling to work."

That didn't stop Trump and other Republicans from treating those goals as fact, suggesting that Democrats want to destroy air travel and expand the welfare rolls.

Republicans also jumped on Ocasio-Cortez's proposal to hike the marginal tax rate to 70 percent as a way to finance her environmental initiative. Even so, Warren followed by suggesting a "wealth tax" on Americans with large fortunes to help finance her child-care plan.

Democrats are "afraid to tell their base what is practical" and instead are offering policies that have little chance of being enacted, said Bryan Lanza, a former campaign aide to Trump who regularly defends the president on cable news.

Recent Democratic presidential nominees such as Clinton, Obama and John Kerry ran as centrists. This is the first election in the modern era, Lanza said, in which progressives "are sucking up all the oxygen and energy."

Democrats as a whole, however, have been moving in a more leftward direction for years. According to Gallup polling, the number of Democrats who identify themselves as "liberal" has risen from 32 percent in 2001 to 46 percent as of 2018.

That shift has largely been among white, highly educated Democrats. African-American and Hispanic voters remain more moderate — which could present a challenge as the party tries to mobilize those groups to vote in greater numbers.

So far, the moderate wing of the party is under-represented in the 2020 field. Some Democratic strategists are concerned the party did not heed the lesson from last year's congressional elections, when it took power in the U.S. House of Representatives largely through moderate candidates who won over suburban voters by focusing on "kitchen-table" issues such as coverage for preexisting medical conditions.

Senator Amy Klobuchar of Minnesota is one of the few Democrats in the presidential field to push back at the progressive agenda. At a CNN town hall this week, she called the Green New Deal "aspirational" and suggested Medicare for all was only a potential long-term goal.

John Delaney, a former Maryland congressman and a centrist who has gotten little traction as a presidential contender, this week said the 2020 primary "is going to be a choice between socialism and a more just form of capitalism."

Antjuan Seawright, a Democratic strategist in the early primary state of South Carolina, said candidates must soon balance sweeping agendas with more pragmatic proposals.

"It has to be a mixed bag of what makes sense and will not cause us long-term political damage,' he said. (Reporting by James Oliphant; Editing by Colleen Jenkins and Cynthia Osterman)

Our Standards: The Thomson Reuters Trust Principles.