WASHINGTON — Rep. Joe Donnelly, a freshman Democrat of Indiana, has a blunt message for the speaker of the house, Rep. Nancy Pelosi: Stick to a “middle-of-the-road agenda” or their party’s control of Congress may last just two years.

If Ms. Pelosi “goes too far one way or another, we’re not coming back,” Mr. Donnelly said.

He sees his party’s victory in the November elections as less an endorsement of its agenda than a rejection of Republican rule: “People just got real tired of this bunch, and they fired them.”

Mr. Donnelly’s view reflects those of many of the 30 House Democrats elected in districts previously held by Republicans.

Their fragile hold on their seats means they’ll be pushing their new speaker, who represents heavily Democratic San Francisco, to limit confrontations with President Bush and the Republicans over taxes, the war in Iraq, stem-cell research, and abortion.

“Pelosi needs to show she can govern,” a professor at American University in Washington who specializes on Congress and the presidency, James Thurber, said. Democrats hold 233 seats in the House, while Republicans have 202. In the Senate, Democrats hold only 51 of the 100 seats — and one Democrat, Senator Johnson of South Dakota, remains hospitalized after brain surgery.

Mr. Donnelly, 51, said he likes most of the legislation Ms. Pelosi is trying to push through in the first 100 hours of Democratic control of the House, including an increase in the minimum wage and a plan to allow Medicare to negotiate the cost of drugs with pharmaceutical companies, both scheduled for votes this week.

It may be more difficult for him to support other parts of the party’s agenda. Mr. Donnelly, a practicing Catholic, opposes abortion and is also against federal funding stem-cell research that uses human embryos, which most Democrats back.

He also may clash with the Democratic leadership over how to handle the war in Iraq.