NEW YORK (CBSNewYork/AP) — The presidential campaign of 2016 is heating up.

On Monday, the Republican field against Hillary Clinton, who officially entered the race Sunday, grew even larger with Florida Sen. Marco Rubio launching his campaign.

So far, Clinton is the only Democratic contender.

She wasted no time putting in the miles on the campaign trail, posting a photo of herself with supporters on Twitter during a pit stop on her way to Iowa.

The battleground state is her first official campaign stop since announcing her bid for the White House.

“I’m running for president,” Clinton said in a two-minute video announcement released Sunday on social media, previewing a platform that will showcase the middle class. “Every day Americans need a champion and I want to be that champion.”

Clinton on Monday also greeted a shocked 19-year-old college student, Chris Learn, at a gas station in Altoona, Pennsylvania.

“She asked me what I was studying in college and I told her criminal justice and she said she was a big supporter of that.”

Meanwhile, Rubio told his top donors on a conference call on Monday that he is running for president because he feels “uniquely qualified” to pitch his Republican Party as one that will defend the American Dream.

Senator Marco Rubio (Drew Angerer/Getty Images News)

Senator Marco Rubio (Drew Angerer/Getty Images News)

The first-term Republican from Florida told his biggest backers that he sees the coming presidential campaign as a choice between the past and the future. In a swipe at Clinton, Rubio said the former first lady “is a leader from yesterday” and that he has always felt the United States is about tomorrow.

Rubio spoke with donors before a flashy political rally set for Monday night in Miami.

He joins GOP Sens. Ted Cruz and Rand Paul, who have already announced their presidential campaigns.

Paul has taken aim at Clinton’s tenure as Secretary of State and her handling of Benghazi.

“I think the fact that she didn’t provide that security will go to the heart of the matter whether or not we should have her as commander in chief,” Paul said on “Face The Nation.”

His team released an ad Monday in four early voting states saying, “Hillary Clinton represents the worst of the Washington machine.”

Analysts said the Republicans joining the race have Democrat fatigue.

“At the end of eight years, the natural tendency in American politics when a party has had the White House for two terms is to look to the other party,” said Ron Brownstein, educational director of the National Journal. “There is an instinct for change.”

And back in New York outside of her downtown Brooklyn campaign headquarters there were some posters of protest, but Clinton has already gained the backing of Gov. Andrew Cuomo, Sens. Charles Schumer and Kirsten Gillibrand.

But as CBS2’s Dick Brennan reported, Mayor Bill de Blasio, who helped run her senate campaign, isn’t offering his support just yet.

“Progressives are demanding from all of our candidates an actual vision on economic change,” de Blasio said.

So has the Clinton campaign already faced down its first double-cross – from de Blasio, a man whom they helped make? Democratic strategist Hank Sheinkopf said absolute loyalty should not be expected.

“There are no political friends,” Sheinkopf said. “There are just people in politics who use each other on a regular basis.

Sheinkopf said DeBlasio will come around eventually.

Sen. Richard Blumenthal, D-Connecticut, however, said Clinton has everything she needs to win the presidency if she relies on her own instincts and drive.

Blumenthal has known the Clintons since the three were students at Yale Law School 40 years ago.

Blumenthal believes Clinton’s major asset is what he calls her “piercing sense of purpose and penetrating intellect.”

“But also a great sense of humor and a perspective about herself that I think will surprise and amaze a lot of people as they get to know her,” Blumenthal told WCBS 880 Connecticut Bureau Chief Fran Schneidau.

Clinton’s team said her stop in Iowa on Monday will be small and low key but she is gearing up for her first large campaign rally sometime next month.