WASHINGTON (Reuters) - President-elect Donald Trump, who insulted rivals during his 2016 campaign with nicknames such as “Crooked Hillary” and “Little Marco,” has a new nemesis: U.S. Senate Democratic leader Chuck Schumer, a man he derides as a “clown.”

U.S. Senate Minority Leader Chuck Schumer (D-NY) speaks with reporters after the weekly Senate Democratic caucus luncheon at the U.S. Capitol on Washington, U.S. January 4, 2017. REUTERS/Jonathan Ernst - RTX2XJZX

The rhetorical banter that erupted between the two New Yorkers this week centered on a fight over Republican plans to dismantle Obamacare, an effort that Trump has put at the top of his domestic agenda.

Just two weeks before he is to be sworn in as the 45th U.S. president, Trump has taken to Twitter to denigrate the hard-charging senator, someone he feted at a fundraiser in 2008 for Democratic senators at the real estate magnate’s posh Florida oceanside estate.

Schumer, whose Democrats are in the minority in the Senate but still have enough muscle to potentially block some of Trump’s legislative initiatives, clashed with the president-elect as Republicans this week took the first steps to try to gut President Barack Obama’s signature healthcare law known as Obamacare.

Schumer used Trump’s “Make America Great” campaign slogan to mock the Republican repeal effort, saying it would “make America sick again.”

On Twitter, Trump shot back that Democrats were to blame for what he called the “failed Obamacare disaster.” He added: “Don’t let the Schumer clowns out of this web.”

On Thursday, Trump wrote on Twitter that Democrats were being led by “head clown Chuck Schumer.”

That prompted the Senate Democratic leader to say that Trump should stop wasting his time calling people names and instead “roll up his sleeves” and come up with a workable substitute for Obamacare.

At campaign rallies last year, Trump attacks on Democratic presidential candidate Hillary Clinton as “Crooked Hillary” stirred enthusiasm among his supporters who chanted “lock her up.”

During the heated primary campaign season, Trump swatted at rival Republican presidential candidates Ted Cruz, Jeb Bush and Marco Rubio with cries of “Lyin’ Ted,” “low-energy” Bush and “Little Marco.”

The mocking may have been unorthodox, but it proved part of a winning formula, sweeping Trump’s opponents off the stage.

Matthew Green, a Catholic University political science professor, said Trump is tangling with a different kind of opponent in Schumer.

“He can name-call Democratic leaders in Congress, but he can’t get rid of them. They still occupy the legislative branch,” Green said.

On the other hand, engaging with Trump carries risks for Schumer, who undoubtedly will feel pressure from the more liberal wing of the Democratic Party if he cuts deals, especially given the ire Trump stirs among millions of Democratic voters.

Trump has outlined an ambitious legislative agenda including overhauling the tax code, a $1 trillion infrastructure investment program and renegotiating major trade deals, while also pumping up U.S. defense spending.

Thomas Quinn, a Democratic activist and long-time Washington lobbyist, said Trump was fortunate to have Schumer as the top Democrat in the Senate, because, like Trump, the senator likes to cut deals and is a somewhat moderate Democrat.

In some ways, Schumer is tailor-made for Trump. Both are bold New Yorkers expert at capturing media attention. However, their paths have rarely crossed until now.

Tony Sayegh, a New York-based Republican campaign strategist whose Jamestown Associates firm did work for Trump, warned that unless Democrats cooperate with Trump, he will use his bully pulpit to win public support.

“Can you picture Air Force One landing in Indiana, in Montana, in western Pennsylvania, eastern Ohio, central Michigan, and northern Wisconsin, and Donald Trump walks out and gets crowds these politicians could never dream of getting?”