President Biden’s decision to drop his reelection bid and Vice President Kamala Harris’s emergence as the front-runner for the Democratic nomination has vulnerable Senate Democrats scrambling to fend off new GOP attacks on her record.

Biden’s biggest vulnerability was his age and doubts about his fitness for office, but the silver lining for Democrats was that Biden’s biggest problems were confined to him as a candidate and did not substantially impact Democratic senators downballot, according to polls.

But now incumbents such as Sens. Jon Tester (D-Mont.), Sherrod Brown (D-Ohio), Bob Casey (D-Pa.) and Jacky Rosen (D-Nev.) are being pressed on Harris’s most controversial policy statements, stretching back to the 2020 Democratic primary.

Democratic senators had worried that lack of voter enthusiasm for Biden would hurt Democratic turnout in Senate battleground states.

Democrats don’t know for sure whether Harris as their nominee for president will drive minority and young voters to the polls in significantly higher numbers than Biden would have in November. But they are generally optimistic about her having an energizing effect on the party, pointing out that she raised $81 million during her first 24 hours as a candidate after Biden dropped out of the race.

The National Republican Senatorial Committee (NRSC) on Monday circulated a memo to Senate Republican campaigns highlighting controversial elements in Harris’s record, ranging from immigration and border policy, to “Medicare for All” and opposition to the Keystone XL pipeline and fracking.

Jason Thielman, executive director of the NRSC, wrote in a memo to campaigns that Harris “creates a strong down-ballot opportunity for Republicans.”

“An endorsement of Kamala Harris is an endorsement of her extreme agenda, and Harris is arguably a bigger threat to Democrats’ Senate majority than Joe Biden,” he wrote.

Of the Democratic incumbents in tough races, Brown, Casey, Rosen and Sen. Tammy Baldwin (D-Wis.) have endorsed Harris for president.

Only Tester hadn’t endorsed Harris as of Monday afternoon.

Tester last week called on Biden not to seek reelection and voiced his support for an open nomination process.

Senate Republicans, however, pointed out that Tester encouraged Harris to run for the Senate in 2015, when he was chair of the Democratic Senatorial Campaign Committee (DSCC).

Senate Republicans strategists are zeroing in on Harris’s vice-presidential record as a “border czar.”

They are highlighting Harris’s opposition while in the Senate to the Trump-era health order known as Title 42, which was used to block migrants from entering the country, and her suggestion that Immigration and Customs Enforcement should be rebuilt “from scratch.”

Senate Republicans say that Harris’s opposition to fracking will be an issue in the Pennsylvania Senate race, where Sen. Bob Casey (D-Pa.) is running comfortably ahead of his Republican opponent, Dave McCormick.

They are also highlighting her past support of the Green New Deal touted by Rep. Alexandria Ocasio-Cortez (R-N.Y.), her praise of a $150 million cut in police funding in Los Angeles and her call to “reimagine” the role of police.

Tommy Garcia, a spokesperson for the DSCC, said that the shake-up atop the Democratic ticket won’t substantially impact Senate races where Republicans are “stuck” with “flawed” candidates.

“Senate Republicans are in meltdown mode because they are still stuck with the same flawed recruits and toxic agenda that have left them trailing Senate Democrats all cycle long. Senate races are candidate vs. candidate battles, and we’ll win because we have the far superior candidates,” the spokesperson said.

Other Democrats say that while Republicans are trying to attack Harris’s statements and positions from 2019 and 2020, when she was running in a crowded primary for the Democratic nomination, she can run on her more moderate record as a member of the Biden administration.

“She’s been in the mainstream of the Democratic Party. When she was attorney general in California, progressives criticized her for being a cop. When she ran for Senate, they said she wasn’t liberal enough. When she ran for president, progressives weren’t really with her,” said Jim Kessler, executive vice president for policy at Third Way, a centrist Democratic think tank.

“What she has is a now three-and-a-half-year record as vice president in which she can talk about helping to bring down crime, murder rates plummeting, border crossings dropping like a stone, an economy that is the envy of the free world,” he said. “That’s the case she can make. That’s the case she’ll have to make.”

Some Democratic senators, however, in recent weeks were reluctant to join the push to boot Biden from the ticket because of the unpredictable impact downballot.   

Senate Majority Leader Chuck Schumer (D-N.Y.) stopped short of endorsing Harris on Monday but said he would meet with her soon.

“She is rapidly picking up support from grassroots delegates from one end of the country to the other. We look forward to meeting in person with Vice President Harris shortly as we collectively work to unify the Democratic Party and the country,” he said in a statement with House Democratic Leader Hakeem Jeffries (N.Y.).

Senators who pushed back against efforts to oust Biden argued to colleagues privately that despite the president’s low approval rating and poor performance in battleground state polls, he wasn’t dragging down Senate Democratic incumbents.

These Biden allies warned that Harris would become the nominee if Biden dropped out and might become a liability for downballot Senate Democratic incumbents, according to a person familiar with internal caucus discussions.  

As of recently, every Democratic incumbent was running ahead of their Republican challenger in internal and high-quality public polls, except for Tester, who’s running neck and neck with Republican Tim Sheehy.  

Now those warnings from two weeks ago are reverberating, as Republican strategists comb through Harris’s record to find ammo to use in Senate races.

The NRSC memo circulated Monday highlighted that Harris was scored by GovTrack.us as the most liberal senator in 2019.

It cited Harris’s role as Biden’s “border czar” and her pledge in the 2020 Democratic presidential primary to seek to decriminalize illegal border crossings, something that most Democratic candidates for president also supported at the time, with the exception of Sen. Michael Bennet (D-Colo.).

It noted her statement in 2018 that Congress should overhaul the nation’s immigration system and maybe “start from scratch” by eliminating the Immigration and Customs Enforcement agency.

It pointed to her support for a pathway to citizenship for millions of illegal immigrants already in the country, which has been a cornerstone of Democratic immigration policy since the Senate passed comprehensive immigration reform in 2013.

Senate Republicans are also citing Harris’s statement in 2019 that, as president, she would support abolishing the filibuster to pass sweeping climate change legislation and her 2019 co-sponsorship of Sen. Bernie Sanders’s (I-Vt.) proposal to expand Medicare to cover all Americans.

They will try to tag Democratic candidates with Harris’s initial endorsement in 2019 for abolishing private health insurance as part of a broader plan to expand Medicare, something she later disavowed. Harris later said she misheard a question about the issue at one of the presidential debates.

Harris’s past positions have caused heartburn for some major Democratic donors, who wanted Biden to stay in the race.

Democratic donor John Morgan announced on the social media platform X that he would not raise money for Harris and told The Hill he thinks she’ll lose in November.

“You have to be enthusiastic or hoping for a political appointment to be asking friends for money. I am neither. It’s others turn now,” Morgan wrote on X.

Morgan said he would have preferred to see Kentucky Gov. Andy Beshear (D) or even Sen. Joe Manchin (W.Va.), who left the Democratic Party to become an independent, head the ticket.