The prospect of a President Kamala Harris facing down a Republican-controlled Senate is coming into focus as she rises in the 2024 contest, even as GOP hopes of capturing the Senate grow because of improving polling in a pivotal Montana race.

A Republican-controlled chamber could thwart Harris’ nominees to fill out her administration and the courts, along with her legislative agenda. Top Senate Republicans told NBC News she would need their sign-off to secure votes on any judicial nominees, including for the Supreme Court. And some Harris supporters worry that without a united Congress, she would struggle to get much done legislatively.

“I honestly believe she has to have both the House and the Senate in order for her to be able to get anything done. Anything at all,” said Frankie Veltri, a retired 77-year-old voter in Goodyear, Arizona. “And if she doesn’t have one or the other, putting more emphasis on the Senate … she will never be able to do anything that she said she would do, and then it will be the same old grind. You know, ‘She made all these promises and she didn’t do them.’ I mean, that’s evident with Biden.”

Sen. John Cornyn, R-Texas, a Judiciary Committee member who is running to be Senate GOP leader when Mitch McConnell retires after this year, said Harris would “absolutely” have to negotiate judicial and Supreme Court nominees with his party if Republicans control the Senate — and not assume they would get votes.

“They’re not going to be able to nominate the most radical people like they would under their current scenario. So if they want them to get any floor time, they would have to enter into a negotiation with us,” Cornyn told NBC News in an interview.

The prospect of a split Congress looms over a possible Harris win even if Democrats have a strong year and sweep every swing state. To capture the Senate, Republicans have to flip just two seats in solidly red states — West Virginia, which Democrats have conceded, and Montana, where Democratic Sen. Jon Tester trails in most polls — while holding seats in GOP-friendly Florida and Texas.

Under that divided-government scenario, Harris would be the first president since 1989 to enter office without her party controlling both chambers of Congress. It would put her in uncharted waters given the polarization between the two parties in recent decades, with no guarantee that any major judicial nominees, Cabinet picks or legislative items would get a vote.

While a split Congress would require the two parties to negotiate on must-pass bills like funding the government and authorizing the Pentagon, Harris’ other legislative pursuits — including Medicare drug price negotiations, cracking down on grocery price gouging and new investments in housing — would be at the mercy of a Republican Party that has aggressively criticized her ideas.

“It’s always a challenge when you’ve got divided government like that,” said Rep. Nanette Barragán, D-Calif.

She said capturing the House would at least help Democrats “set the agenda” for a Harris presidency. But, she added, “it definitely poses a challenge if we don’t have the Senate.”

Charlie Veltri, the husband of Frankie and an Arizona voter, projected confidence that Harris at the top of the ticket will help Democrats down the ballot. But he said what happens in Montana, which could be the tipping-point Senate race, affects everyone.

“I think about [Democratic Sen.] Jon Tester and the fight he’s going through, and some of the other people. And it’s not enough just to have Ruben [Gallego, the Democratic congressman and Arizona Senate nominee] win. We’ve got to have the rest of them, because if we don’t, we don’t control the Senate,” he said. “It’s not enough just to win the presidency. She’s got to have control of the Senate, at the very minimum.”

On the Supreme Court, it’s unclear when another vacancy will occur. But history suggests Harris would get at least one — the last president who didn’t get any new justices was Jimmy Carter. And if it’s a GOP-led Senate, Republicans would control a floor vote.

“Particularly with the judiciary, because we have the power of confirmation, I think they’re going to have to think long and hard about who they submit and whether or not they think they could get them cleared through the Senate,” said Senate Minority Whip John Thune, R-S.D., who is also running for leader.

Sen. Chuck Grassley, R-Iowa, would have seniority to be the next Judiciary Committee chair and said he intends to claim the position if Republicans win control. He declined to say how he would handle picks for judges and justices in a potential Harris administration.

“I’m not going to presume doom and gloom, but needless to say it’ll be more difficult” to confirm judges in a Republican-led Senate, said Sen. Richard Blumenthal, D-Conn., a Judiciary Committee member.

Asked if he worries about a repeat of Merrick Garland in 2016, when a GOP Senate refused to give President Barack Obama’s nominee a vote, Blumenthal said, “I worry about it, but there’s a limit to how irresponsible they can be.”

Sen. Sheldon Whitehouse, D-R.I., another Judiciary Committee member, said, “I don’t know that they’d confirm none, but they’d certainly make it an obstacle course for her.”

A Harris campaign spokesman declined to comment when asked about the importance of the Senate, the prospect of divided government and how she would seek to work with Republicans if they win control.

Henry Wade, a Maricopa, Arizona, City Council member, said he’s “confident but concerned” about Democrats’ prospects of holding the Senate.

“I think that they can do it. … I know I’m out working hard, canvassing and talking to people and passing out signs,” he said. “We’ll see what happens. I’m going to keep doing my part.”

Louis Olivas, a retired professor in Tempe, Arizona, acknowledged that Republicans could “potentially” stop Harris’ Supreme Court nominees if they took the Senate. But he said he’s “not worried at all” about them stonewalling, because “she’s a great negotiator.”

“They’re not going to shut her down for four years,” Olivas said

Sahil Kapur reported from Washington and Alex Tabet from Phoenix.