WASHINGTON — A measure to cut child poverty nearly in half, another to raise the minimum wage to $15 an hour, a third to end job discrimination against LGBTQ workers — Democrats have a far-reaching legislative agenda, but the threat of Senate filibusters has already constrained their choices.

The first sacrifice to the Senate rules came last week as the chamber’s parliamentarian ruled that President Biden’s proposed minimum wage increase did not fit into the narrow category of spending and tax measures that can evade filibusters. The ruling came the same day that the House passed the Equality Act, a measure to prohibit discrimination against LGBTQ Americans, which has previously been blocked in the Senate by Republican filibusters.

Still to come are at least two more measures expected to pass the House — a voting rights bill and a measure to reform police practices — which core Democratic constituencies ardently want, but which face grim chances in the Senate because of the power the minority has under the rules to block the majority.

The prospect that a large part of their agenda could go nowhere, even though their party controls both houses of Congress and the White House, has angered many Democratic activists. They’ve put abolition of the filibuster into the spotlight to a degree not seen for nearly half a century. A book denouncing the filibuster by Adam Jentleson, who was an aide to former Senate Democratic leader Harry Reid of Nevada, has become required reading for progressive Democrats.

So far, Biden and key Democratic senators, notably Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona, have resisted changing the rules. In the 50-50 Senate, Democrats need agreement of their entire caucus to act.

But as White House officials like to note, the Biden presidency is barely a month old. As time passes and bills don’t, pressure will grow.

No one ever planned the filibuster.

As scholars have shown, the men who wrote the Constitution intended the Senate to work by majority rule, as the House does. Over the decades, however, the Senate’s tradition of allowing lengthy speeches evolved into one in which debate could continue without end, blocking legislation.

In 1917, the Senate adopted a rule requiring a two-thirds vote to end debate. Over the next half-century, filibusters, enforced with marathon speeches, became the prime tool for Southern senators to block civil rights bills.

In 1975, the Senate adopted a rule that allowed 60 senators to end debate. Reformers expected that change would weaken the filibuster’s power. But an unexpected thing happened — the weaker filibuster became more common.

Senators began threatening extended debate on so much legislation that by the early 2000s, the 60-vote threshold had become a de facto requirement for nearly everything of consequence.

That led to several moves to further curtail filibusters. In 2013, a Democratic-majority Senate changed the rules to allow most nominations to pass with a simple majority. In 2017, a Republican-controlled Senate expanded that exemption to include Supreme Court nominations. Even earlier, in 1980, Congress agreed that certain spending and tax measures could pass the Senate by a simple majority vote under a special process known as budget reconciliation.

The resulting messy set of rules and exemptions governs the Senate today, giving extensive power to the parliamentarian, a theoretically neutral staff member who sorts through the chamber’s precedents and, among other things, determines what fits within the filibuster exemptions.

That’s why the proposal to cut child poverty by revamping the existing child tax credit has a very strong chance of passing: It fits within the budget rules as part of the current $1.9-trillion COVID-19 relief package and has unanimous Democratic support.

Winning that expansion would mark a major accomplishment — “the most important thing I’ve done in my political career,” Sen. Sherrod Brown (D-Ohio), one of the main sponsors, said on a call with reporters Thursday.

Expanding the credit to cut child poverty in half “will result in a 1:8 return on investment,” said Wes Moore, chief executive of Robin Hood, a New York-based anti-poverty organization.

That’s based on a study by the National Academy of Sciences, which found that over time, cutting child poverty in half would expand the U.S. economy by up to $1 trillion a year through increased productivity, lower health costs, reduced crime and other benefits.

But because of the way the rules work, Democrats will expand the child credit for just one year in the current bill. They’ll need to do it all over again later this year to make the change stick.

Other party priorities face an even more complex path. After the parliamentarian’s ruling Thursday that the $15 minimum wage wasn’t allowed under budget reconciliation rules, Sens. Bernie Sanders (I-Vt.) and Ron Wyden (D-Ore.), who head the Budget and Finance committees, respectively, said they may try a work-around that would raise the minimum wage for at least the nation’s largest companies. Their proposal would impose a tax on big employers that pay workers under $15 per hour.

But if they want to raise the wage for all, Democrats will likely need to pass a separate measure with 60 senators. They currently don’t have the votes.

For measures like the Equality Act or voting rights, which don’t involve the federal budget, the 60-vote hurdle is more intractable.

Democrats have a range of choices. Senate Majority Leader Charles E. Schumer of New York could bring a series of civil rights bills to the floor, for example, and dare Republicans to show up and talk them to death the old-fashioned way. The majority could also change the rules to carve out more exceptions or to further lower the vote requirement for ending debate, without going all the way to majority rule.

A lot of older Democratic lawmakers, including Biden, are loath to take those steps. They’ve seen filibusters block Republican legislation and argue that the rules can protect both sides.

Many younger Democratic lawmakers and activists disagree: The filibuster helps Republicans far more than Democrats, they argue, because it has given the GOP the power to control the agenda even though the party’s lawmakers represent a minority of the nation’s population.

So far, Biden has been able to mostly stay out of that argument. His COVID-19 package is moving toward passage — the House approved it Friday, and the Senate will start debate this week. Knocking the minimum wage out of the package actually improved the bill’s odds since that was one of the few parts of the bill that lacked unanimous Democratic support.

However, avoiding the filibuster issue won’t work forever.

Republicans have shown little reluctance to use the rules to the fullest. Whether the test comes on the Equality Act, voting rights, policing or some other priority, at some point in this Congress, Democrats will face a stark choice: Limit filibusters or accept defeat.