WASHINGTON — Over the past week, Representative Elise Stefanik of New York, the third-ranking House Republican, has blasted President Biden for providing infant formula to undocumented immigrants while “American mothers” suffer amid a nationwide formula shortage.

She has attacked Democrats and “pedo grifters,” borrowing language from the baseless pro-Trump QAnon conspiracy theory that claims there is a Satan-worshipping cabal of liberal pedophiles, which has evolved into a movement on the right.

And after the deadly mass shooting in Buffalo, where a heavily armed white man is accused of killing 10 Black people at a supermarket in a racist rampage, Ms. Stefanik is under scrutiny for campaign advertisements she has circulated that play on themes of the white supremacist “great replacement” theory. That belief, espoused by the Buffalo gunman, holds that the elite class, sometimes manipulated by Jews, wants to “replace” and disempower white Americans.

Last year, in an ad on Facebook, Ms. Stefanik accused “radical Democrats” of planning what she described as a “PERMANENT ELECTION INSURRECTION.”

“Their plan to grant amnesty to 11 MILLION illegal immigrants will overthrow our current electorate and create a permanent liberal majority in Washington,” the ad said.

Ms. Stefanik, a onetime moderate Republican who worked in President George W. Bush’s White House and was a protégé of former Speaker Paul D. Ryan, has long been seen as a rising star in her party, and she still is. But as she has ascended, the Republican Party has transformed, lurching to the right along with her district in upstate New York, and she has shape-shifted along with it.

Now, she proudly describes herself as an “ultra MAGA” warrior and aggressively appeals to the hard right, sounding nativist themes that animate the Republican base.

The racist massacre, which unfolded in her own state, has shone a spotlight on Ms. Stefanik. In the days since, Democrats and even some Republicans have suggested that Ms. Stefanik and her party have stoked the beliefs that led to the killings, by catering to a base that will not tolerate outright condemnation of the most fringe ideas.

Representative Liz Cheney, Republican of Wyoming, who was ousted last year as conference chair and replaced by Ms. Stefanik, said on Monday that House Republican leaders had “enabled white nationalism, white supremacy, and antisemitism.” In a posting on Twitter, she called on her party’s leaders to “renounce and reject these views and those who hold them.”

But as Democrats decried the white supremacist ideology that gave rise to the mass killing, Ms. Stefanik and other House Republican leaders were largely silent about the racism that apparently motivated the shooter.

Far from apologizing for the nativist language and themes she has amplified, Ms. Stefanik, who has been floated to former President Donald J. Trump as a potential running mate and who is widely seen in Congress as a candidate to become her party’s next House whip, is following Mr. Trump’s example.

Like Mr. Trump, Ms. Stefanik’s response when under fire is to attack her attackers. Like Mr. Trump, she vehemently defends herself against charges of holding any racist views, while at the same time using rhetoric that energizes far-right and fringe groups.

On Monday, she released a lengthy statement attacking the media for reporting on statements she has made that echo replacement theory claims, but never disavowed the ideology, and did not condemn racism or white supremacy.

Later, asked in a brief exchange whether she would disavow or repudiate replacement theory, Ms. Stefanik did not, saying: “I condemn any form of racism.”

Aides to Ms. Stefanik said the Facebook advertisement being criticized was addressing the need for stronger border security, and referring to Mr. Biden’s proposal to offer a pathway to U.S. citizenship for nearly 11 million undocumented people and a proposal to give 800,000 noncitizens in New York the right to vote in municipal elections.

And they said her reference to “pedo grifters” had not been to QAnon, but to John Weaver, a Never Trump operative who had made sexual overtures to young men, and whose former colleagues Ms. Stefanik blames for dubbing her #EliseStarvefanik on Twitter after her complaints about undocumented infants receiving formula.

Democrats were quick to point out that attacks like those Ms. Stefanik have lodged have at their core the same grievance as replacement theory.

“The subtext is clear,” Senator Chuck Schumer, Democrat of New York and the majority leader, said in a speech. “These hard-right MAGA Republicans argue that people of color and minority communities are somehow posing a threat — a threat — to the American way of life.”

Ms. Stefanik has also refrained from calling out extremists in her party’s ranks who are explicit about such views. Earlier this year, she refused to denounce Representative Marjorie Taylor Greene, Republican of Georgia, after Ms. Greene spoke at a white nationalist event.

“They represent their constituents, and they are held accountable for the statements they make,” Ms. Stefanik said in an interview in March, saying that the House Republican conference as a whole did not share Ms. Greene’s views.

Ms. Stefanik, a Harvard graduate who once privately conceded to friends that Mr. Trump was a liability for her party, has embraced her role as one of his unequivocal supporters. After Mr. Biden derided Republicans who have refashioned themselves in the mold of Mr. Trump as “ultra MAGA,” Ms. Stefanik turned the criticism into a fund-raising opportunity. Her campaign now sells T-shirts that read, “I am ultra MAGA and proud of it.”

The approach suggests that Ms. Stefanik sees little downside in being viewed as extreme; the only political risk she fears is being regarded as insufficiently hard-line. In an interview on the sidelines of the Republican retreat outside Jacksonville in March, Ms. Stefanik repeatedly echoed Mr. Trump’s lies about the 2020 election being stolen and refused to acknowledge Mr. Biden as the legitimately elected president.

Afterward, an aide followed up to reinforce that she had repeated the false election claims and refused to accept Mr. Biden’s legitimacy, making sure those points were not lost on the reporter.

Still, while Ms. Stefanik has aggressively courted the far right, she has also worked to maintain a veil of respectability within her party, working to remain palatable to donors.

To ensure a future in the Republican Party, said Bill Kristol, the prominent Never-Trump Republican, “you want The Wall Street Journal to feel OK about supporting you, but you also want networks like O.A.N.N. and hosts like Tucker Carlson saying, ‘Yeah she’s pretty good.’ ”

Stephen K. Bannon, the former Trump White House official who now hosts an influential podcast on the right, said Ms. Stefanik could not care less about criticism from the left, using an expletive for emphasis.

What keeps her up at night, Mr. Bannon said, is any threat from the right.

“She’s in a competition right now with Representative Jim Banks about who is going farther right,” he said, referring to the Indiana Republican and chairman of the Republican Study Committee, who has also refashioned himself from a movement conservative into a Trump acolyte as he seeks to rise in power in Washington.

Some of Ms. Stefanik’s recent moves, people close to her said, appeared to be motivated by her internal competition with Mr. Banks.

Last week, Mr. Banks was one of 56 Republicans who voted against a $40 billion package of aid for Ukraine. Ms. Stefanik voted for the bill, drawing criticism from the right for doing so.

Ms. Stefanik had reservations, said people familiar with her thinking, and raised concerns about the optics of sending so much money abroad amid so many challenges at home. But she ultimately sided with the majority of the conference, and with the Republican leader, Kevin McCarthy, and voted for the bill.