Scalise, now No. 2 on the Republican leadership ladder, edged rival Jim Jordan in a secret ballot vote to secure his party's nomination for the top post.
Lawmakers had been told to expect a 3pm vote on Wednesday, but that hour came and went with no action after several of Jordan's backers said they would not support Scalise in a vote on the House floor.
Republicans can afford only a handful of defections as they control the chamber by a narrow 221-212 majority and Democrats are not expected to vote for any Republican candidate.
"We're working hard to get unified. We need to get unified because the world is not waiting," Scalise, 58, told reporters.
"Obviously we still have work to do."
Acting Speaker Patrick McHenry told reporters a vote "could be as early as today".
"It's not over yet. That was the inside race. Now the outside race starts," said Representative Thomas Massie, who had voted for Jordan. Massie and said he expected to meet privately with Scalise ahead of a floor vote.
The tally was 113 for Scalise and 99 for Jordan, lawmakers said.
Jordan plans to vote for Scalise on the floor and is encouraging his colleagues to do the same, according to a source who spoke on condition of anonymity.
McCarthy in January had to endure 15 rounds of voting before clinching the speakership.
Before selecting Scalise, Republicans rejected a proposal that would have required their nominee to secure 217 of their 221 votes. That allowed Scalise to win the nomination quickly, but left open the question of whether he would face a repeat of the battle in January for the role of speaker, which played out over four days.
Republican Representative Kat Cammack predicted the floor vote would be a "bloodbath ... a continuation of the chaos that has plagued the House".
Republicans say they need to quickly resolve a leadership vacuum that has prevented the House from addressing the war in Israel, approving more aid to Ukraine and passing spending bills before current government funding runs out on November 17.
Scalise said his first action as speaker would be to schedule a vote on a resolution making clear that the United States stands with Israel in its war with Hamas militants in Gaza.
It took only eight Republicans to oust McCarthy last week, a fact that could make leading the caucus a challenge for any new speaker.
While McCarthy was the first US speaker to be removed in a formal vote, the last two Republicans to hold the job wound up leaving under pressure from party hardliners.
Americans have little confidence in Congress' ability to overcome its partisan differences and the Republican infighting that led to McCarthy's ouster on October 3.
Some 64 per cent of respondents to a Reuters/Ipsos poll last week said they did not believe Washington politicians could put aside partisan disagreements for the good of the nation.
