WASHINGTON — House Speaker Paul Ryan said Wednesday he was trying to be as constructive as possible as he looked forward to a Thursday meeting with presumptive Republican presidential nominee Donald Trump aimed at unifying the fractured party.

But Ryan, the most high-profile Republican who has not endorsed Trump, warned that bringing party factions together would take some time after a grueling primary season. This suggested there might not be instant results from his get-together Thursday with Trump and Republican National Committee Chairman Reince Priebus.

Trump’s takeover of the Republican Party has shaken the party’s establishment and prompted soul-searching over whether to reluctantly get behind him or cede any role in the Nov. 8 presidential election, when Hillary Clinton is expected to be his Democratic opponent.

Trump has the potential to appeal to a greater number of Republican voters but must make changes to make party leaders more comfortable with him, Republican officials and lawmakers say.

“I think he has to show what kind of president that he would be,” said Sen. Susan Collins, R-Maine. “But I believe he can do that, so I am not one who has foreclosed the possibility of eventually supporting him. But I need to see more from him.”

Trump faces pressure to tone down his rhetoric and clarify his policy positions as a step toward unifying the divided party. Some Republicans were alarmed at his recent talk of raising taxes on the wealthy.

But Ryan also faces some pressure to smooth things over after saying last week that he was not yet ready to support Trump, an extraordinary statement that was seen as keeping some distance from the presumptive nominee and leaving open his prospects for running for president in 2020.

“What we are trying to do is to be as constructive as possible, to have a real unification,” Ryan told reporters Wednesday after a meeting with his fellow House Republicans.

“After a tough primary, that’s going to take some effort. We are committed to putting that effort in,” said Ryan, who said he does not really know Trump.

“I want to be a part of that unifying process so that we are at full strength this fall, so that we can win this election. We cannot afford to lose this election to Hillary Clinton,” Ryan said.

Ryan, 46, a conservative congressman since 1999 who ran unsuccessfully for vice president in 2012, and Trump, a 69-year-old billionaire real estate developer, are very different Republicans who do not see eye to eye on many things, from immigration to trade.

But Ryan said Wednesday these differences are not at issue, because “this is a big-tent party. There is plenty of room for different policy disputes in this party.”

One lawmaker at Wednesday’s House Republican gathering raised a request for Trump to meet with all Republicans in the House, a source who was in the room said, asking not to be named. Such a meeting is expected to happen sometime before the July 18-21 Republican convention in Cleveland, the source said.

On Tuesday night, Ryan spoke by phone for about 16 minutes with a Trump emissary, former Republican presidential candidate Ben Carson, according to Carson spokesman Armstrong Williams.

Some Republicans have been rankled by Trump’s policy proposals, including his declaration that NATO is obsolete and his call to temporarily ban Muslims from entering the country.

But some House lawmakers who back Trump expressed their displeasure with Ryan for withholding his support at a small group meeting with the speaker later Wednesday, an aide to one of the lawmakers said.

After Ryan said last week he was not yet ready to support Trump, the candidate fired back that he was not ready to support Ryan’s agenda. But on Wednesday, Trump had warm words for Ryan and said he thought they were doing fine.

“We’ll see what happens” at the meeting, Trump told Fox News. “If we make a deal, that’ll be great. If we don’t, we’ll trudge forward like I’ve been doing.”