(Updates with Thursday vote on $1 trillion infrastructure bill)

By Humeyra Pamuk

WASHINGTON, Sept 26 (Reuters) - U.S. House of Representatives Speaker Nancy Pelosi on Sunday set a vote on the $1 trillion bipartisan infrastructure bill for Thursday and voiced confidence it would pass.

Debate on the legislation, which passed the Senate with Democratic and Republican support on Aug. 10 and would help fund road, bridge, airport, school and other construction projects, will begin on Monday, she added.

Pelosi has not yet set a date to bring to the floor a larger, $3.5 trillion social welfare and climate bill - whose cost has divided her fellow Democrats - and said it is still under negotiation.

She added it was "self-evident" that the larger spending bill might shrink in size.

"We are now working together with the Senate and the White House on changes to this historic legislation," she added.

Both measures are key to Democratic President Joe Biden's economic agenda.

"Tomorrow, September 27, we will begin debate on the Bipartisan Infrastructure Framework on the Floor of the House and vote on it on Thursday, September 30, the day on which the surface transportation authorization expires," Pelosi said in a letter to Democrats that was released by her office.

Earlier on Sunday, Pelosi said she would not bring the infrastructure bill to a vote until she was sure it would pass, but expressed confidence about its prospects.

"Let me just say that we're going to pass the bill this week," she told ABC News' "This Week" anchor George Stephanopoulos.

Asked at the White House on Sunday whether Pelosi had the votes she needed, Biden said: "I'm optimistic about this week. It's going to take the better part of the week, I think," he told reporters.

HIGH-STAKES WEEK

Democrats have so far failed to reach consensus on the timing of the two measures.

Some progressive lawmakers insist the $1 trillion infrastructure bill be held back until the bigger measure is ready. Moderates want the infrastructure bill enacted whatever the progress on the larger package, which includes provisions for expanding healthcare for children and the elderly and for investing in steps to drastically reduce emissions blamed on climate change.

Pelosi did not specifically address how the divisions within the Democratic Party would be bridged, but said the final figure for the larger measure would be lower than $3.5 trillion.

"That was the number that was sent to us by the Senate and by the president. Obviously with negotiation, there has to be some changes in that the sooner the better, so that we can build our consensus to go forward," she said.

The House Budget Committee advanced the larger bill on Saturday, reporting the legislation with a favorable recommendation.

Pelosi was speaking at the start of a high-stakes week for both parties in Washington. Congress is fast approaching a Thursday deadline to continue funding federal agencies or face the second partial government shutdown in three years.

"We have to make sure we keep the government open and we will," Pelosi said.

In late October or early November, the U.S. Treasury will run out of money to pay its obligations, meaning the government faces the risk of a historic default if Congress does not act.

The Senate will hold a procedural vote on Monday evening on legislation that has already passed the House to fund the U.S. government through Dec. 3 and suspend the nation's borrowing limit until the end of 2022.

Republicans say they oppose the bill because it includes a suspension of the debt limit, and while they oppose allowing the U.S. government to default, they want Democrats to suspend the debt limit without their votes.

Pelosi accused Republicans of being irresponsible and said the failure to extend the debt ceiling could have a wider impact on the U.S. economy. "This is beyond a big deal," she said. (Reporting by Humeyra Pamuk; Additional reporting by Jeff Mason and Arshad Mohammed; Editing by Grant McCool, Diane Craft and Peter Cooney)