Democratic Rep. Chris Pappas is warning his party that Republicans could win New Hampshire’s open U.S. Senate seat if national Democrats don't invest heavily in the increasingly competitive race against Republican John E. Sununu.

Pappas, a four-term congressman seeking to succeed retiring Democratic Sen. Jeanne Shaheen, has increased his fundraising appeals as the contest draws growing attention from both parties. “This election could go either way,” Pappas said in one recent appeal.

The nonpartisan Cook Political Report moved the race from Lean Democrat to Toss Up on Sept. 15. The change followed polling showing a much closer contest than earlier surveys and increased Republican spending behind Sununu, a former senator who represented New Hampshire from 2003 to 2009.

Sununu is also benefiting from a prominent political family in New Hampshire. His father, John H. Sununu, served as governor and White House chief of staff under President George H.W. Bush, while his brother, Chris Sununu, served four terms as governor.

Outside spending has become a major factor. The National Republican Senatorial Committee has allocated $5.3 million in coordinated spending for Sununu, while broader Republican groups have committed substantially more to advertising in the state. Pappas has raised considerably more directly through his campaign, but Republicans have sought to close that gap through outside spending.

The candidates are emphasizing different issues. Sununu has focused on affordability, including housing, health care, energy and grocery prices, while Pappas has sought to connect Sununu to President Donald Trump and criticize administration policies over costs and other issues.

Democrats have held New Hampshire’s Senate seat since 2009, but the 2026 contest comes as the party seeks to defend several seats while attempting to gain the additional seats needed for Senate control. Pappas’ warning reflects the growing national attention on a state that has traditionally favored Democrats in federal races but has also shown a willingness to elect Republicans to statewide office.