The Republicans Are Headed to Milwaukee. So Are Protesters.

As tens of thousands of Republicans arrive in Milwaukee on Monday for their party’s convention, another large gathering is expected just outside the official meeting site: Thousands of left-leaning protesters, some of whom have spent the last two years preparing for this moment, will march in opposition to the Republicans’ platform and their nominee, Donald J. Trump.

More than 120 groups have joined a Coalition to March on the R.N.C. 2024, and organizers say they expect as many as 5,000 people to take to the streets of downtown Milwaukee for a march and rally on Monday.

Organizers said they have seen a surge of interest and intensity in the protests outside the Republican National Convention as questions mounted in recent weeks over President Biden’s age and acuity.

“With all the developments with Biden, that makes Trump a much more imminent threat, and that’s going to increase our numbers,” said Omar Flores, a protest coalition leader. “People see the urgency in taking this on.”

Activists said they will focus not on a single issue but on many and have described a range of causes motivating them to join the main planned protest on Monday. Some said they objected to the United States’ support of Israel, a rise in bans on abortion in Republican-led states and policies aimed at preventing migrants from entering the country. Many said they also were seeking more accountability by police officers and protections for L.G.B.T.Q. people.

One goal in planning for the convention has been to increase the number of activists and grow stronger ties between groups on the left, said Alan Chavoya, of the Milwaukee branch of the Alliance Against Racist and Political Repression.