The National Republican Senatorial Committee (NRSC) on Monday and Tuesday released new attack ads against former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and Sens. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.) as the three White House contenders cement their status in the Democratic primary field’s top tier.

The ads, released on Facebook, come ahead of the third Democratic presidential primary debate in Houston, which will feature 10 candidates vying for the opportunity to take on President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE next year.

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The ads against Warren ask Facebook users who they would vote for between Trump and the Massachusetts Democrat, showing a fluctuating bar graph between the two.

The ads against Sanders say the self-avowed Democratic Socialist would work to “undo all the great work President Trump has done if he becomes president” and “turn America into a socialist country.”

Going after Biden, the NRSC also asks voters who they would pick between Trump and the former vice president and says a “vote for Joe Biden is a vote for four years of Obama’s leftist policies.”

Some interesting @NRSC Facebook ads up right now leaning into a Biden/Trump general election matchup



Also one that flashes the message: "Biden 2020 = Four More Years of Obama" pic.twitter.com/V85OGeLSEq — Johnny Verhovek (@JTHVerhovek) September 10, 2019

Biden has sought to tie himself to former President Obama during his campaign, hoping Obama's lingering popularity among the Democratic base will help propel the former vice president to the party’s 2020 nomination. But Obama served as a chief boogeyman for the GOP during his eight years in office.

The ads against the three Democrats come as polling shows them extending their leads over the rest of the pack, with Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.) and South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBillionaire who donated to Trump in 2016 donates to Biden The Hill's Morning Report - Sponsored by Facebook - GOP closes ranks to fill SCOTUS vacancy by November Buttigieg stands in as Pence for Harris's debate practice MORE, the nearest two contenders, stagnating behind them.

Biden has maintained a narrowing lead among the top three candidates as Sanders has remained largely steady and Warren, buoyed by her litany of detailed policy platforms, has surged in national and statewide surveys.

Several reports have said that Trump and his campaign view Biden as their toughest potential opponent, fearing the centrist former vice president could chip into Trump’s appeal to blue-collar workers in key Rust Belt states that helped hand him the White House in 2016.