Washington (CNN) Massachusetts Sen. Elizabeth Warren entered the 2020 presidential race this week in a strong financial position as a prodigious fundraiser with lots of available cash.

Warren, who announced a presidential exploratory committee on Monday, has $12.5 million left over from her 2018 Senate campaign that she can transfer to her presidential campaign coffers. Only one other Democratic senator considering a presidential bid has come close to Warren's early war chest: New York's Kirsten Gillibrand, who ended the 2018 election season with $10.5 million remaining in her Senate campaign account.

Warren, Gillibrand and the other Democrats weighing presidential bids will need every cent they can collect. For starters, President Donald Trump, who took the unprecedented step of filing for re-election on the day he was sworn into office in January 2017, already has raised a whopping $106 million for his reelection.

And presidential campaigns are notoriously expensive affairs. The price tag of the 2012 general election contest between former President Barack Obama and Republican Mitt Romney -- including money from the outside groups backing their campaigns -- topped $2.3 billion, according to a tally by the non-partisan Center for Responsive Politics.

Here's a look at the early fundraising picture for some of the possible contenders:

Massachusetts Sen. Elizabeth Warren

Warren, a former Harvard law school professor turned consumer advocate, is known for her searing criticism of Wall Street banks. So, no, don't expect New York financiers to line up to raise money on her behalf. She's also ruled out help from a super PAC.

"I don't think we ought to be running campaigns that are funded by billionaires, whether it goes through super PACs or their own money that they're spending," Warren told reporters this week.

She enters the field with a sizable small-donor base: More than $6 of every $10 Warren raised from individual contributors for her 2018 Senate re-election came from donors who contributed in amounts of $200 or less, Federal Election Commission records show.

Warren will need to lean on those kinds of donors, who can contribute repeatedly to her campaign before hitting contribution limits, to sustain her in a crowded Democratic field.

She already is racing to collect cash for the presidential exploratory committee she announced on New Year's Eve. A campaign email sent in the waning hours of 2018 urged supporters to make donations as small as $3.

A key liberal activist, Adam Green, co-founder of the Progressive Change Campaign Committee, immediately endorsed Warren's bid. He said her record of building coalitions to achieve policy goals, such as the creation of the watchdog Consumer Financial Protection Bureau following the 2008 financial crisis, gives her credibility with liberal donors.

"Donations to Elizabeth Warren are not an impulse buy," Green told CNN this week. "They are fundamentally built on a track record of her consistently challenging power -- whether that's challenging Wall Street and big insurance or challenging systemic racism."

New York Sen. Kirsten Gillibrand

Gillibrand, raised nearly $18 million for her Senate re-election campaign and coasted to victory over a little-known rival. That's left her with a hefty war chest of leftover cash -- $10.5 million.

As a senator representing the nation's financial capital, she also has ties to deep-pocketed donors. The securities and investment sector ranks among the top five industries supporting Gillibrand in recent years, according to the Center for Responsive Politics.

She's campaigned for other Democrats in the midterms and invested heavily in digital advertising on Facebook -- a move that will help her expand her name recognition and build lists of potential supporters.

But Gillibrand ran afoul of some powerful Democratic donors after leading calls urging Minnesota Democrat Al Franken to resign from the Senate. Franken quit last year following allegations that he touched women inappropriately.

Liberal billionaire George Soros told The Washington Post last June that he blamed Gillibrand for pushing out Franken "in order to improve her chances."

New Jersey Sen. Cory Booker

The New Jersey senator wasn't on the ballot in 2018, but he crisscrossed the country in an effort to influence the midterm elections -- ranging from Georgia, where he stumped for Stacey Abrams, who ran unsuccessfully for governor, to the key presidential caucus state of Iowa.

Booker, who has served in the Senate since October 2013, has extensive political and financial networks around the country and counts the people who work in the securities industry as among the top sectors donating to his campaign, according to the Center for Responsive Politics' analysis of donations.

And Booker drew national attention long before he became a senator as part of a Sundance Channel documentary series, "Brick City," which launched in 2009 and centered on his role as then-mayor of Newark, New Jersey. Facebook CEO Mark Zuckerberg, one of the planet's richest men, pledged $100 million to the city's schools during Booker's tenure.

Steve Phillips, an influential Democratic donor in San Francisco, already has launched a pro-Booker super PAC, Dream United, and said he plans to raise $10 million to support a Booker presidential campaign.

Vermont Sen. Bernie Sanders

Sanders' army of grassroots supporters helped propel the Vermont independent senator into the No. 2 spot in the 2016 Democratic presidential primary.

His presidential campaign had raised some $228 million before Sanders conceded to Hillary Clinton in July of that year.

Small-dollar donors turned up again to help fund his re-election to a third Senate term in 2018. More than three-quarters of Sanders' individual contributions came in small amounts, and he ended the 2018 election with solid $8.8 million in the bank.

But with Warren and other progressives likely to enter the race, can Sanders still command small-donor devotion from liberal activists?

Rufus Gifford, who served as finance director of Obama's re-election campaign in 2012, is skeptical.

Sanders "did almost nothing to raise money in 2016," Gifford wrote on Twitter this week. "Because of the overwhelming level of grassroots support, he didn't have to. Question is: Is it still there? Can that strike twice? I am doubtful."

Sean Bagniewski, head of the Iowa's Polk County Democrats, said the online, small-donor fundraising techniques that Sanders pioneered in the 2016 campaign have grown commonplace among Democrats. And donors and activists are making fresh decisions about whom to support.

"I don't think the bases are staying with anybody in this next race. I know Bernie people who like Kamala and Cory," Bagniewski said, referring to Sens. Kamala Harris and Cory Booker of California and New Jersey, respectively. "I know Hillary Clinton people who like Elizabeth Warren."

Outgoing Texas Rep. Beto O'Rourke

The Texas Democrat emerged as the breakout fundraising star of the 2018 elections, collecting more than $80 million in his unsuccessful US Senate campaign against Republican incumbent Ted Cruz.

But the campaign drained his resources. O'Rourke ended the election season with just $477,000 remaining in his campaign's bank account.

Nearly half of O'Rouke's individual contributions came from small donors. And he's already attracting attention from some of the party's biggest fundraisers. Chicago financier and former US ambassador to the UK, Louis Susman, a fundraiser so successful that one of his hometown papers once called him a vacuum cleaner for his ability to suck up campaign dollars, recently told CNN that he would back O'Rourke should the Texan enter the race.

A decade ago, that mix of establishment bundlers and small-donor enthusiasm helped Obama, then a freshman U.S. senator, defy the odds and build a fundraising juggernaut that defeated Clinton in the 2008 Democratic primary.

O'Rourke's backers aren't waiting for him to announce his candidacy. One "Draft Beto" group has established an ActBlue escrow account and is working to raise $1 million for his potential campaign. Another staged a "day of action" on his behalf last month in the early voting state of New Hampshire.

While O'Rourke has drawn early comparisons to Obama's 2008 candidacy, one member of Obama's cabinet has already taken a formal step toward a 2020 race. Julian Castro, who ran the Housing and Urban Development department in Obama's administration, launched an exploratory committee last mont h.

A formal announcement of his decision is expected Jan. 12 in his hometown of San Antonio, where he once served as mayor.

But Castro has not run for office outside of San Antonio and must work to establish a broader base of financial support. A political action committee he established in 2017 has raised about $490,000.

California Sen. Kamala Harris

The California senator was not on the ballot in 2018, but she has raised big sums just two years into her Senate term.

Harris collected nearly $7 million in her Senate campaign account. And about three quarters of her individual contributions came in small amounts, one of the highest proportions among Democratic senators considering running for the presidency in 2020.

Harris also has raised more money for her leadership PAC than any other senator, according to a tally by a nonpartisan think tank, Issue One. (Leadership PACs are separate from candidates' principal campaign committees and provide a vehicle for politicians to raise money in larger increments than allowed by their campaigns. Harris' Fearless for the People PAC has collected $2.3 million.)

Harris has deployed her campaign dollars to build a national profile and expand her lists of potential supporters.

Through the end of December, for instance, her campaign had spent more than $1.2 million on Facebook ads, according to an online database maintained by the social-media giant. A recent Harris ad on Facebook decried the December decision by a Texas judge to invalidate the Affordable Care Act and asked users to help "defend health care" by providing their first names, email addresses and ZIP codes.

A reshuffled 2020 primary schedule that has pushed California's voting into early March offers a possible strategic advantage to Harris as the home-state senator.

But advertising across the populous state's multiple media markets will be expensive. And Harris could face competition from several other Californians, including Rep. Eric Swalwell who represents parts of the Bay Area, Los Angeles Mayor Eric Garcetti and San Francisco billionaire Tom Steyer

Ohio Sen. Sherrod Brown

The Ohio Democrat raised a whopping $22.7 million in his Senate campaign, but he spent heavily to beat Republican Jim Renacci in a state Trump won by 8 percentage points in 2016.

Brown ended the election with $1.8 million in leftover funds -- not an ideal financial base to quickly build the infrastructure needed for a presidential campaign. But Brown's getting credit for waging a successful battle in a red-leaning state.

Gifford, the Obama fundraiser, put Gillibrand, Brown and another possible contender, Minnesota Sen. Amy Klobuchar, in the same category.

These senators are "unlikely to match the big names" in fundraising, Gifford tweeted. "But it will be a tortoise/hare thing for them. Need to peak at the right time and totally doable."

Minnesota Sen. Amy Klobuchar

Klobuchar raised far less in her 2018 re-election campaign -- about $8 million -- than Gillibrand and Brown did in theirs.

Klobuchar ended the midterm election season with nearly $4.4 million banked in her campaign account -- just ahead of Booker's $4.1 million cash balance.

Former Vice President Joe Biden

The former vice president -- who has toured the country promoting his book, "Promise Me, Dad," and stumping for fellow Democrats -- would enter the race as the early frontrunner. He led the field of potential contenders in a recent CNN/Des Moines Register/Mediacom poll of likely Iowa caucusgoers

And Biden can draw on extensive ties to Democratic fundraisers forged during a Senate career that began in 1973 and as part of an Obama-Biden team that raised a combined $1.5 billion over two presidential elections.

Since leaving office in January 2017, Biden also has built a network of nonprofits and academic institutions staffed by former aides and close allies, likely enabling him to quickly assemble a campaign infrastructure.

Meanwhile, his political action committee, American Possibilities, raised more than $2.5 million since it was launched in 2017 and doled out more than $500,000 to Democratic candidates and committees during the 2018 midterms.

In recent interviews, a number of Obama's top financial backers told CNN they would help collect campaign contributions for Biden should he enter the race. But Biden, who would be 77 on Election Day 2020, may not be a shoo-in for everyone in the Obama crowd.

Stonyfield Farm co-founder Gary Hirshberg, a top-tier Obama bundler, said Biden likely would have won had he entered the race in 2016, but he believes Biden now is "too old" to seek the presidency.

Hirshberg told CNN he was looking mostly closely at supporting Brown or another possible 2020 contender, outgoing Colorado Gov. John Hickenlooper.

The self-funders

And then there are the candidates who will need no financial help at all should they jump into the race.

At least two billionaires , former New York mayor Michael Bloomberg and Steyer, an environmental activist and former hedge-funder, are weighing their own presidential bids.

And they each unloaded more than $100 million into the midterms. Steyer focused heavily on driving youth turnout. Bloomberg and his team directed millions of dollars worth of advertising to two dozen House races to help Democrats successfully seize control of the chamber.

Another self-funder, departing Maryland Democratic Rep. John Delaney, already has declared his White House candidacy and has poured $3.5 million of his own money into the long-shot bid.