Senator Kyrsten Sinema, the moderate Arizona Democrat who has objected to the size and scope of President Biden’s sweeping legislative agenda, has been trailed all over the country by progressive activists seeking to pressure her. They have followed her into a bathroom in Arizona, on an airplane and even to the Boston Marathon.

But this week, with the Senate out of session, those activists would have had to travel even farther to press their case with Ms. Sinema in person — she’s in Europe on a fund-raising trip.

A spokesman for Ms. Sinema said she had participated in fund-raising for the Democratic Senatorial Campaign Committee but declined to say where or provide any additional details. One person briefed on the matter said an event had occurred in Paris. It was not clear whether her trip to Europe was at the urging of the party committee.

The chairman of the Democratic Senatorial Campaign Committee, Senator Gary Peters of Michigan, is also in Europe this week and headlined a dinner on Wednesday in London, with contribution levels as much as $36,500, according to a copy of an invitation. Ms. Sinema’s name does not appear on that invitation.

A spokesman for the Senate committee declined to comment on the events in Europe or on Ms. Sinema’s role.

Although political campaigns and parties cannot raise money from foreign nationals, American citizens living abroad can and do regularly contribute.

Ms. Sinema’s office declined to say how long she would be abroad, what countries she was visiting, how the trip was being paid for and whether she was doing any additional fund-raising for her own campaign. Her political team had reached out to set up meetings in London and Paris, according to two people familiar with the matter.

John LaBombard, a spokesman for Ms. Sinema, said she remained engaged in the negotiations in Washington.

“So far this week, Senator Sinema has held several calls — including with President Biden, the White House team, Senator Schumer’s team, and other Senate and House colleagues — to continue discussions on the proposed budget reconciliation package,” Mr. LaBombard said. “Those conversations are ongoing.”

Ms. Sinema is one of the most closely watched Senate Democrats in the country as one of two centrists, along with Senator Joe Manchin of West Virginia, who have objected to the price of Mr. Biden’s budget package, which could spend as much as $3.5 trillion over a decade remaking social and climate policy in the nation.

With the Senate tied 50-50, Democrats need unanimity to pass any package, which has greatly empowered both Mr. Manchin and Ms. Sinema as the party’s vocal holdouts. Vice President Kamala Harris would serve as the tiebreaking vote.

Ms. Sinema has pushed for the House to adopt a separate, bipartisan $1 trillion infrastructure bill while the larger budget package is being negotiated. She called the delay in voting on the infrastructure package “inexcusable” and “deeply disappointing” in early October.

When the Senate is out of session, senators are free to fund-raise, return to their home states for events, remain in Washington working on legislation or travel abroad, both for official government business and for vacations.

Ms. Sinema’s fund-raising while in the midst of budget negotiations has already drawn scrutiny. In late September, a group of five business lobbying groups, many of which oppose the budget bill, held a fund-raiser for her in Washington, D.C. Sponsors of that event included the National Association of Wholesaler-Distributors and the grocers’ political action committee, as well as lobbyists for roofers and electrical contractors.

Days later, Ms. Sinema traveled to Arizona where she had a “retreat” for her political action committee at a high-end resort and spa in Phoenix. When she left Washington, the reason that her spokesman provided was a medical appointment for a foot injury. That injury prevented her from running the Boston Marathon, though she still attended. Some climate activists showed up and waved a banner that urged her to “Be brave — fight for us.”

Progressive activists have expressed growing frustration with Ms. Sinema in recent weeks, with some trying to recruit a primary challenger when she is next up for re-election, in 2024.

Ms. Sinema entered July with $3.5 million in her re-election account.