Even lesser-known prospective Democratic 2020 presidential hopefuls are already facing a withering barrage of criticism from the Republican establishment, foreshadowing a historically fierce and competitive campaign season as higher-profile potential nominees like former Vice President Joe Biden and California Sen. Kamala Harris wait in the wings.

Former Housing and Urban Development (HUD) Secretary Julián Castro and Hawaii Rep. Tulsi Gabbard -- widely considered longshot candidates with relatively little name recognition -- have also come under fire from the left in the days after they formally declared they would run, as Democrats hope to find a candidate who can realistically unseat President Trump.

“Julián Castro has made history by becoming one of the biggest lightweights to ever run for president," Republican National Committee (RNC) spokesman Michael Ahrens said in a statement shortly after Castro announced his candidacy surrounded by reporters in his hometown San Antonio, Texas on Saturday. "He was a weak mayor who couldn’t even handle being HUD secretary. This is obviously just another desperate attempt to become someone else’s running mate.”

Castro, 44, was the mayor of San Antonio for five years before serving in the Obama administration for less than three. But the RNC, as part of a comprehensive response to Castro's candidacy, has issued rapid-response documents pointing to contemporaneous reporting that has characterized Castro's government service as wholly unremarkable.

San Antonio's mayorship was a part-time position paying only $3,000 a year and roughly equivalent to a city councilmember role when Castro held the job, and the city's executive leadership was almost entirely conducted by a separate official who acted as city manager -- an inconvenient fact for Castro that was noted not only by the GOP this week, but also by left-leaning Vox.com in 2016.

Dylan Matthews, writing for Vox in 2016, asserted that Castro would be a poor vice presidential pick because he "simply is not qualified to be one heartbeat away from the presidency."

"Picking Castro would look like Clinton selecting a Dan Quayle–like lightweight, who lacks the requisite policy knowledge and experience to assume the presidency should something happen to her," Matthews argued.

Also in 2016, Politico reported in an unflattering profile that Castro didn't want to lead the Department of Homeland Security because he was worried about the political ramifications of a terrorist attack on his watch. Citing several sources, Politico reported that once at HUD, Castro would "freeze up" whenever "detailed policy questions about housing and mortgage financing came up," and seemed "not interested enough in HUD policy to want to engage deeply."

In his speech Saturday, Castro, a Stanford University and Harvard Law School graduate, emphasized his goals of universal health care and fighting climate change more than his credentials. "I’m running for President because it’s time for new leadership and to make sure opportunities I had are available to every American," he said.

But, separate from the GOP-led attacks, Castro's limited name recognition posed a readily identifiable and immediate challenge to his bid.

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“People here in San Antonio know him obviously -- but, if you get out of Texas, and you ask an ordinary citizen on the street, and you say, Julian Castro, they’ll probably say, who?” Johnny Arredondo, a member of the Republican Club of Bexar County, told Fox News this weekend.

Washington Free Beacon reporter Alex Griswold wrote on Twitter Monday that it "only took about a day" for everyone to move on from Julian Castro, amid reports that New York Democratic Sen. Kirsten Gillibrand might announce the formation of a presidential exploratory committee on Tuesday.

The 37-year-old Gabbard, meanwhile, has also come under political attacks more typically endured by frontrunners mid-campaign cycle. Gabbard announced her presidential bid in an interview Friday.

Republicans -- and many Democrats -- quickly focused on Gabbard's support for Syrian President Bashar al-Assad, whose government lawmakers have accused of war crimes and even genocide during that country's brutal civil war. Gabbard met with Assad secretly in 2017 without first informing top Democrats, and has dismissed Assad's opponents as "terrorists."

Gabbard has also made headlines for referring to President Trump as "Saudi Arabia's b----," after the president argued for the importance of a strong U.S-Saudi relationship amid calls for him to take a tougher stance on the kingdom in response to the killing of writer Jamal Khashoggi.

“Tulsi Gabbard has an even bigger problem than her lack of experience – it’s that she has no base of support," an RNC spokesman said. "Liberals think she’s too conservative, conservatives think she’s too liberal, and just about everyone thinks her coziness with Bashar al-Assad is disturbing.”

Gabbard, an Iraq War veteran, is known for being politically unpredictable, something she has publicly said she takes pride in. She famously resigned as Democratic National Committee (DNC) vice chair in 2006 to endorse Vermont Independent Sen. Bernie Sanders' presidential bid. (Sanders, another potential 2020 contender, has been besieged in recent days by accusations that he failed to clamp down on sexual misconduct allegations against some of his staffers during that campaign.)

But some of Gabbard's most fiery remarks against same-sex marriage in years past, as well as her active campaign to promote “traditional marriage” legislation, have drawn intense scrutiny from the left.

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In years past, she several times referred to LGBTQ activists as “homosexual extremists,” a view that echoes that of her father, Mike Gabbard, whom a New Yorker profile of the congresswoman described as having long been “Hawaii’s leading opponent of the gay-rights movement, an energetic and often brusque activist who stood ever ready to denounce what he called ‘the radical homosexual agenda.’”

"So, she’ll co-sponsor bills supporting LGBTQ rights, but still personally thinks we’re icky," wrote Parker Malloy, the editor-at-large for the liberal media watchdog group Media Matters for America, on Twitter. "In any case, it’s really weird to see self-described leftists line up to support her instead of more progressive candidates. That’s all."

Gabbard, who has apologized for her previous comments, was a visible force against same-sex marriage, and in 2004 spearheaded a fight in the state against a same-sex union measure. “To try to act as if there is a difference between ‘civil unions’ and same-sex marriage is dishonest, cowardly and extremely disrespectful to the people of Hawaii,” she said at the time. “As Democrats, we should be representing the views of the people, not a small number of homosexual extremists.”

Gabbard then also blasted Massachusetts’ passage of same-sex marriage legislation, making it the first in the nation to recognize gay marriage. Speaking on behalf of the Alliance for Traditional Marriage and Values, a group headed by her father, she said the Massachusetts marriage law would cause a ripple effect across the country.

“It is highly likely that federal judges will soon be tearing apart our U.S. Constitution in order to force same-sex marriage down the throats of the people of Hawaii and America,” Gabbard said. “The only way to protect traditional marriage in Hawaii and throughout our country, the only way to stop activist federal judges from rewriting our constitution, is by the passage of the Federal Marriage Amendment.”

Another lesser-known declared presidential candidate, businessman and former Maryland Rep. Democratic Rep. John Delaney, has largely escaped the harshest criticism from both parties, in part because of his status as a dark horse among dark horses. On ABC's "This Week" on Sunday, Delaney -- who has called for universal health care and reversing Trump's tax cuts -- touted the importance of bipartisanship, rather than immediately achieving progressive political goals.

The first 100 days of a presidency, Delaney said, are more about proving "to the American people that we can actually start solving problems and getting things done," rather than passing sweeping new progressive legislation.

But campaigning in Cedar Rapids, Iowa, on Saturday, Delaney drew a crowd of only about 50, according to local reports, and he faced explicit questioning about his electability.

Should Delaney's profile rise, however, he can likely expect to face the no-holds-barred approach that GOP leaders have taken against virtually every other known White House contender -- including the more prominent ones, like Massachusetts Sen. Elizabeth Warren.

"If Elizabeth Warren, often referred to by me as Pocahontas, did this commercial from Bighorn or Wounded Knee instead of her kitchen, with her husband dressed in full Indian garb, it would have been a smash!" Trump wrote on Twitter Sunday evening, referring to his frequent criticism of Warren for previously claiming Native American ancestry during her academic career based on scant evidence.

Warren, seeking to blunt that line of attack, took a DNA test late last year that showed she may have had a distant Native American ancestor -- an inconclusive result that angered many Democrats and Native American groups, including the Cherokee Nation, who charged that it was inappropriate and even racist to use a DNA test to assert tribal heritage.

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Trump then took a shot at Warren's decision to livestream herself drinking a beer in her kitchen on New Year's Eve, in a move reminiscent of similar Instagram broadcasts by younger Democratic firebrands like New York Democratic Rep. Alexandria Ocasio-Cortez and former Texas Senate candidate Beto O'Rourke. At one point in the livestream, Warren turned to her husband and thanked him for his presence.

"Best line in the Elizabeth Warren beer catastrophe is, to her husband, “Thank you for being here. I’m glad you’re here.,'" Trump wrote. "It’s their house, he’s supposed to be there!"

Fox News' Madeline Rivera and Alex Pappas contributed to this report.