Sens. Cory Booker (D-N.J.) and Kirsten Gillibrand (D-N.Y.) during the Senate Environment and Public Works Committee confirmation hearing on the nomination of Scott Pruitt to be administrator of the Environmental Protection Agency on Jan. 18. (Michael Reynolds/European Pressphoto Agency)

There’s a pretty good chance that the future of the Democratic Party wasn’t far behind President Trump during Friday’s inauguration.

Within 50 to 100 feet of Trump, members of the U.S. Senate watched him take the oath of office. Inside that elite club are as many as a half-dozen Democrats who are considered ambitious rising stars, the type who might end up winning their party’s nomination four years from now.

One by one, each of these potential candidates put their heads down Friday and vowed to get to work challenging Trump’s administration. As she headed to the confirmation votes on some of Trump’s Cabinet nominees, Sen. Elizabeth Warren (Mass.) carried a laptop in hand and ducked into an elevator with a reporter.

How had she spent the first hours of Trump’s reign? “Working,” she said. “I went back to my office and sat and worked.”

Sen. Cory Booker (N.J.) began his day by pouring out his political soul in a 500-word Facebook post, which by 6 p.m. had been shared nearly 4,000 times. “This is not a time to curl up, give up or shut up,” Booker began. “It is time to get up; to stand up, to speak words that heal, help, and recommit to the cause of our country.”

Later in the day, Sen. Kirsten Gillibrand (N.Y.) forced a roll call on Trump’s new defense secretary, James N. Mattis, so that she could cast the lone dissenting vote against his nomination because the retired general had left the Marine Corps a few years ago, breaking the tradition of keeping that post in the hands of someone who had been out of the military for at least seven years.

Those are just a few of the Democrats who might run for the nomination. None were preparing for a 2020 presidential bid. Instead, until the results came in Nov. 8, these up-and-coming senators expected Hillary Clinton to defeat Trump and run for reelection in 2020, making 2024 the earliest possible national campaign for them.

Trump’s victory has sped up the timeline — they’ll have to decide on running a presidential campaign in a little more than two years, needing to put together an operation that can run in the Iowa caucuses, probably in January 2020, and then all across the nation.

There is no guarantee that the Democrats will choose a senator as their next presidential nominee, particularly now that Trump, with no prior political experience, broke all the way through to win the presidency as a businessman outsider.

But in the past 16 elections, the party not holding the White House has nominated six sitting senators to be their nominee, two former vice presidents who had served in the Senate, seven governors — and Trump.

Add to that the fact that Democrats now have few prominent governors. California’s Jerry Brown is the highest-profile Democratic governor, but he is 78.

One wild card in the Democratic sweepstakes is former vice president Joe Biden, himself a 36-year veteran of the Senate. Now 74, Biden would be 77 on Election Day 2020.

But the former vice president and Michelle Obama are the only two Democratic political figures with high approval ratings who are constitutionally capable of being president. Biden, if he continues to float a candidacy, would soak up a lot of attention in the next couple of years. (The former first lady has repeatedly said she has no interest in elective politics.)

Despite Biden’s recent toying with the idea, many party strategists doubt he would ultimately run.

That probably makes the Senate, with its direct role in overseeing the Trump White House, the big proving ground for Democrats wanting to build a national profile to make the run.

Along with Booker, Gillibrand and Warren, Sen. Tim Kaine (Va.), who was Clinton’s vice-presidential nominee, and Sens. Amy Klobuchar (Minn.) and Sherrod Brown (Ohio) are among those mentioned as potential candidates. Also not to be overlooked is Sen. Kamala Harris (Calif.), who is not even three weeks into her first term but is only the second African American woman to be elected in her own right to the Senate.

A former state attorney general, Harris recently hired two highly touted communications staffers from Clinton’s Brooklyn-based campaign to help build her media image. She was one of the first Democrats to announce that she would oppose the confirmation of John F. Kelly to be homeland security secretary, basing her opposition not on the popular retired Marine general’s credentials but instead on Trump’s immigration policies.

While Kelly sailed to confirmation by a vote of 88 to 11, Harris was joined by Warren, Booker and Gillibrand in voting against him.

Brown and Klobuchar, both elected in 2006, are the most senior members of this crop of potential candidates; the rest have served fewer than eight years in the Senate. Those two, along with Kaine, Gillibrand and Warren, must first win reelection in 2018 before they can truly think about mounting a presidential bid.

Once a moderate member of the House from the Hudson Valley, Gillibrand has transformed herself into a liberal firebrand. She, Harris, Klobuchar and Booker participated in Saturday’s Women’s March on Washington. Warren took part in the march in Boston.

Booker, a master of social media’s many platforms, and Warren, known for her fiery anti-Wall Street ­speeches, have to broaden their profiles if they want to be considered as commander in chief. So this month, Booker joined the Foreign Relations Committee, and Warren secured a spot on the Armed Services Committee.

“It is a new day,” Booker wrote on Facebook. “We love our country; we will serve it, defend it, and never stop struggling to make its great promise real for all. And no one gets a vote on that.”